diff --git a/modules/openapi-generator/src/main/resources/csharp/ApiClient.mustache b/modules/openapi-generator/src/main/resources/csharp/ApiClient.mustache
index 91ea70055b0..ec45cbab3c5 100644
--- a/modules/openapi-generator/src/main/resources/csharp/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp/ApiClient.mustache
@@ -384,6 +384,14 @@ namespace {{packageName}}.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/modules/openapi-generator/src/main/resources/csharp/api.mustache b/modules/openapi-generator/src/main/resources/csharp/api.mustache
index fae106702fd..527434ba1ab 100644
--- a/modules/openapi-generator/src/main/resources/csharp/api.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp/api.mustache
@@ -290,6 +290,7 @@ namespace {{packageName}}.{{apiPackage}}
             };
 
             var localVarContentType = {{packageName}}.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -375,7 +376,7 @@ namespace {{packageName}}.{{apiPackage}}
             {{/isArray}}
             {{/isFile}}
             {{^isFile}}
-            localVarRequestOptions.FormParameters.Add("{{baseName}}", {{packageName}}.Client.ClientUtils.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}})); // form parameter
+            localVarRequestOptions.FormParameters.Add("{{baseName}}", {{#isPrimitiveType}}{{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}){{/isPrimitiveType}}{{^isPrimitiveType}}localVarMultipartFormData ? {{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}) : {{packageName}}.Client.ClientUtils.Serialize({{paramName}}){{/isPrimitiveType}}); // form parameter
             {{/isFile}}
             {{/required}}
             {{^required}}
@@ -397,7 +398,7 @@ namespace {{packageName}}.{{apiPackage}}
                 {{/isArray}}
                 {{/isFile}}
                 {{^isFile}}
-                localVarRequestOptions.FormParameters.Add("{{baseName}}", {{packageName}}.Client.ClientUtils.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}})); // form parameter
+                localVarRequestOptions.FormParameters.Add("{{baseName}}", {{#isPrimitiveType}}{{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}){{/isPrimitiveType}}{{^isPrimitiveType}}localVarMultipartFormData ? {{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}) : {{packageName}}.Client.ClientUtils.Serialize({{paramName}}){{/isPrimitiveType}}); // form parameter
                 {{/isFile}}
             }
             {{/required}}
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/AuthApi.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/AuthApi.cs
index 2b34a89ac85..649e8375cfe 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/AuthApi.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/AuthApi.cs
@@ -274,6 +274,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -412,6 +413,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/BodyApi.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/BodyApi.cs
index 159ebbd2542..4b91b3612e5 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/BodyApi.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/BodyApi.cs
@@ -663,6 +663,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +793,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -932,6 +934,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1078,6 +1081,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1218,6 +1222,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1352,6 +1357,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1486,6 +1492,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1620,6 +1627,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1754,6 +1762,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1888,6 +1897,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/FormApi.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/FormApi.cs
index 03ef6de434e..45025c63f81 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/FormApi.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/FormApi.cs
@@ -366,6 +366,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -543,7 +545,7 @@ namespace Org.OpenAPITools.Api
                 localVarRequestOptions.HeaderParameters.Add("Accept", localVarAccept);
             }
 
-            localVarRequestOptions.FormParameters.Add("marker", Org.OpenAPITools.Client.ClientUtils.Serialize(marker)); // form parameter
+            localVarRequestOptions.FormParameters.Add("marker", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(marker) : Org.OpenAPITools.Client.ClientUtils.Serialize(marker)); // form parameter
 
             localVarRequestOptions.Operation = "FormApi.TestFormObjectMultipart";
             localVarRequestOptions.OperationIndex = operationIndex;
@@ -682,6 +684,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/HeaderApi.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/HeaderApi.cs
index 41b07bcf1fc..ad1e9009e3c 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/HeaderApi.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/HeaderApi.cs
@@ -261,6 +261,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/PathApi.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/PathApi.cs
index a1ae25b575a..a4a1ff7fc10 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/PathApi.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/PathApi.cs
@@ -267,6 +267,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/QueryApi.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/QueryApi.cs
index 5b5dd8318d5..7c25c5d572f 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/QueryApi.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Api/QueryApi.cs
@@ -691,6 +691,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1005,6 +1007,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1163,6 +1166,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1347,6 +1351,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1483,6 +1488,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1621,6 +1627,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1759,6 +1766,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1897,6 +1905,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2035,6 +2044,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Client/ApiClient.cs
index 655b101ecf7..8fab398a3f8 100644
--- a/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/echo_api/csharp-restsharp/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -384,6 +384,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Api/MultipartApi.cs b/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Api/MultipartApi.cs
index d5af0aa89c0..955220dfbae 100644
--- a/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Api/MultipartApi.cs
+++ b/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Api/MultipartApi.cs
@@ -343,6 +343,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -497,6 +498,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -511,12 +513,12 @@ namespace Org.OpenAPITools.Api
             localVarRequestOptions.FormParameters.Add("status", Org.OpenAPITools.Client.ClientUtils.ParameterToString(status)); // form parameter
             if (marker != null)
             {
-                localVarRequestOptions.FormParameters.Add("marker", Org.OpenAPITools.Client.ClientUtils.Serialize(marker)); // form parameter
+                localVarRequestOptions.FormParameters.Add("marker", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(marker) : Org.OpenAPITools.Client.ClientUtils.Serialize(marker)); // form parameter
             }
             localVarRequestOptions.FileParameters.Add("file", file);
             if (statusArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("statusArray", Org.OpenAPITools.Client.ClientUtils.Serialize(statusArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("statusArray", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(statusArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(statusArray)); // form parameter
             }
 
             localVarRequestOptions.Operation = "MultipartApi.MultipartMixed";
@@ -657,6 +659,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Client/ApiClient.cs
index 91b7422b713..c8d1224a347 100644
--- a/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/others/csharp-complex-files/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -384,6 +384,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/PetApi.cs
index a268ca3af4d..3498fe14e99 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/PetApi.cs
@@ -591,6 +591,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -764,6 +765,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -943,6 +945,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1123,6 +1126,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1297,6 +1301,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1449,6 +1454,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1625,6 +1631,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1813,6 +1820,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/StoreApi.cs
index 27d50bc9bdc..7b969283939 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/UserApi.cs
index 3f99b38d1ed..5cf6eaa5291 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -719,6 +720,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -871,6 +873,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1025,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1175,6 +1179,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1329,6 +1334,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1474,6 +1480,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1623,6 +1630,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/ApiClient.cs
index 38ba17f24b2..6fa812fa08c 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -385,6 +385,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
index 9ccd159a44f..1060a9cbc23 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
index 54f3d2e41a9..ddcafaaeb04 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -399,6 +399,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -667,6 +669,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -793,6 +796,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -919,6 +923,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
index a61c5d240e2..b1d361a676d 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -1245,6 +1245,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1374,6 +1375,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1508,6 +1510,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1642,6 +1645,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1778,6 +1782,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1913,6 +1918,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2039,6 +2045,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2165,6 +2172,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2298,6 +2306,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2440,6 +2449,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2590,6 +2600,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2744,6 +2755,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2920,6 +2932,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3200,6 +3213,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3237,7 +3251,7 @@ namespace Org.OpenAPITools.Api
             }
             if (enumFormStringArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("enum_form_string_array", Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("enum_form_string_array", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(enumFormStringArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
             }
             if (enumFormString != null)
             {
@@ -3415,6 +3429,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3600,6 +3615,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3742,6 +3758,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3892,6 +3909,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4095,6 +4113,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4316,6 +4335,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
index b0e5066eef8..07c4ad398c6 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
index 8db5fd36849..7e74501d82f 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
@@ -641,6 +641,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1024,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1234,6 +1237,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1440,6 +1444,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1599,6 +1604,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1804,6 +1810,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1992,6 +1999,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2189,6 +2197,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
index e8882b0fc2e..af79df6916f 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
index 174d0572e67..700610558ef 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -709,6 +710,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -851,6 +853,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -992,6 +995,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1135,6 +1139,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1289,6 +1294,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1434,6 +1440,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1573,6 +1580,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
index a0a43ea74ae..30158fdf49b 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -385,6 +385,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
index 9ccd159a44f..1060a9cbc23 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
index 54f3d2e41a9..ddcafaaeb04 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -399,6 +399,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -667,6 +669,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -793,6 +796,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -919,6 +923,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
index a61c5d240e2..b1d361a676d 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -1245,6 +1245,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1374,6 +1375,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1508,6 +1510,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1642,6 +1645,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1778,6 +1782,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1913,6 +1918,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2039,6 +2045,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2165,6 +2172,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2298,6 +2306,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2440,6 +2449,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2590,6 +2600,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2744,6 +2755,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2920,6 +2932,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3200,6 +3213,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3237,7 +3251,7 @@ namespace Org.OpenAPITools.Api
             }
             if (enumFormStringArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("enum_form_string_array", Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("enum_form_string_array", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(enumFormStringArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
             }
             if (enumFormString != null)
             {
@@ -3415,6 +3429,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3600,6 +3615,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3742,6 +3758,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3892,6 +3909,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4095,6 +4113,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4316,6 +4335,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
index b0e5066eef8..07c4ad398c6 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
index 8db5fd36849..7e74501d82f 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
@@ -641,6 +641,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1024,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1234,6 +1237,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1440,6 +1444,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1599,6 +1604,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1804,6 +1810,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1992,6 +1999,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2189,6 +2197,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
index e8882b0fc2e..af79df6916f 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
index 174d0572e67..700610558ef 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -709,6 +710,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -851,6 +853,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -992,6 +995,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1135,6 +1139,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1289,6 +1294,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1434,6 +1440,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1573,6 +1580,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
index a0a43ea74ae..30158fdf49b 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -385,6 +385,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Api/FakeApi.cs
index 38d8dec2a29..621402b02c8 100644
--- a/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -269,6 +269,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Client/ApiClient.cs
index 841ef8cadec..61c2f259c1f 100644
--- a/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net6/ParameterMappings/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -383,6 +383,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/AnotherFakeApi.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
index 9ccd159a44f..1060a9cbc23 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/DefaultApi.cs
index 54f3d2e41a9..ddcafaaeb04 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -399,6 +399,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -667,6 +669,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -793,6 +796,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -919,6 +923,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeApi.cs
index 8fc96497aa0..0d9db708d63 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -1245,6 +1245,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1374,6 +1375,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1508,6 +1510,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1642,6 +1645,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1778,6 +1782,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1913,6 +1918,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2039,6 +2045,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2165,6 +2172,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2298,6 +2306,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2440,6 +2449,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2590,6 +2600,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2744,6 +2755,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2920,6 +2932,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3200,6 +3213,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3237,7 +3251,7 @@ namespace Org.OpenAPITools.Api
             }
             if (enumFormStringArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("enum_form_string_array", Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("enum_form_string_array", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(enumFormStringArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
             }
             if (enumFormString != null)
             {
@@ -3415,6 +3429,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3600,6 +3615,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3742,6 +3758,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3892,6 +3909,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4095,6 +4113,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4316,6 +4335,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
index b0e5066eef8..07c4ad398c6 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/PetApi.cs
index 848cd61975b..9769e375ee7 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/PetApi.cs
@@ -641,6 +641,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1024,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1234,6 +1237,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1440,6 +1444,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1599,6 +1604,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1804,6 +1810,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1992,6 +1999,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2189,6 +2197,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/StoreApi.cs
index e8882b0fc2e..af79df6916f 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/UserApi.cs
index 174d0572e67..700610558ef 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -709,6 +710,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -851,6 +853,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -992,6 +995,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1135,6 +1139,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1289,6 +1294,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1434,6 +1440,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1573,6 +1580,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/ApiClient.cs
index 8431e531175..33238956646 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -384,6 +384,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
index 9ccd159a44f..1060a9cbc23 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
index 54f3d2e41a9..ddcafaaeb04 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -399,6 +399,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -667,6 +669,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -793,6 +796,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -919,6 +923,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
index 8fc96497aa0..0d9db708d63 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -1245,6 +1245,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1374,6 +1375,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1508,6 +1510,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1642,6 +1645,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1778,6 +1782,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1913,6 +1918,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2039,6 +2045,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2165,6 +2172,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2298,6 +2306,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2440,6 +2449,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2590,6 +2600,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2744,6 +2755,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2920,6 +2932,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3200,6 +3213,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3237,7 +3251,7 @@ namespace Org.OpenAPITools.Api
             }
             if (enumFormStringArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("enum_form_string_array", Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("enum_form_string_array", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(enumFormStringArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
             }
             if (enumFormString != null)
             {
@@ -3415,6 +3429,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3600,6 +3615,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3742,6 +3758,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3892,6 +3909,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4095,6 +4113,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4316,6 +4335,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
index b0e5066eef8..07c4ad398c6 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
index 96d72132622..4692618e20d 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
@@ -641,6 +641,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1024,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1234,6 +1237,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1440,6 +1444,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1599,6 +1604,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1804,6 +1810,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1992,6 +1999,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2189,6 +2197,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
index 57193a85d29..50765779d94 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
index 174d0572e67..700610558ef 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -709,6 +710,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -851,6 +853,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -992,6 +995,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1135,6 +1139,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1289,6 +1294,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1434,6 +1440,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1573,6 +1580,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
index 8431e531175..33238956646 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -384,6 +384,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Api/DefaultApi.cs
index fddeca8791c..40964553f7f 100644
--- a/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -227,6 +227,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Client/ApiClient.cs
index bc3f8ca8e98..ea3384d5c5f 100644
--- a/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/UseDateTimeForDate/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -383,6 +383,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/AnotherFakeApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
index 9ccd159a44f..1060a9cbc23 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/DefaultApi.cs
index 54f3d2e41a9..ddcafaaeb04 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -399,6 +399,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -667,6 +669,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -793,6 +796,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -919,6 +923,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeApi.cs
index a61c5d240e2..b1d361a676d 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -1245,6 +1245,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1374,6 +1375,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1508,6 +1510,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1642,6 +1645,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1778,6 +1782,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1913,6 +1918,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2039,6 +2045,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2165,6 +2172,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2298,6 +2306,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2440,6 +2449,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2590,6 +2600,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2744,6 +2755,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2920,6 +2932,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3200,6 +3213,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3237,7 +3251,7 @@ namespace Org.OpenAPITools.Api
             }
             if (enumFormStringArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("enum_form_string_array", Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("enum_form_string_array", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(enumFormStringArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
             }
             if (enumFormString != null)
             {
@@ -3415,6 +3429,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3600,6 +3615,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3742,6 +3758,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3892,6 +3909,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4095,6 +4113,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4316,6 +4335,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
index b0e5066eef8..07c4ad398c6 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs
index 086ecc41298..91056b351ad 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs
@@ -641,6 +641,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1024,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1234,6 +1237,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1440,6 +1444,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1599,6 +1604,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1804,6 +1810,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1992,6 +1999,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2189,6 +2197,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/StoreApi.cs
index 57193a85d29..50765779d94 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/UserApi.cs
index 174d0572e67..700610558ef 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -709,6 +710,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -851,6 +853,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -992,6 +995,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1135,6 +1139,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1289,6 +1294,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1434,6 +1440,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1573,6 +1580,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/ApiClient.cs
index a0a43ea74ae..30158fdf49b 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -385,6 +385,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }
 
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
index 9ccd159a44f..1060a9cbc23 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/AnotherFakeApi.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
index 54f3d2e41a9..ddcafaaeb04 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/DefaultApi.cs
@@ -399,6 +399,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -532,6 +533,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -667,6 +669,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -793,6 +796,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -919,6 +923,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
index a61c5d240e2..b1d361a676d 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -1245,6 +1245,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1374,6 +1375,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1508,6 +1510,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1642,6 +1645,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1778,6 +1782,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1913,6 +1918,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2039,6 +2045,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2165,6 +2172,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2298,6 +2306,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2440,6 +2449,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2590,6 +2600,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2744,6 +2755,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2920,6 +2932,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3200,6 +3213,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3237,7 +3251,7 @@ namespace Org.OpenAPITools.Api
             }
             if (enumFormStringArray != null)
             {
-                localVarRequestOptions.FormParameters.Add("enum_form_string_array", Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
+                localVarRequestOptions.FormParameters.Add("enum_form_string_array", localVarMultipartFormData ? Org.OpenAPITools.Client.ClientUtils.ParameterToString(enumFormStringArray) : Org.OpenAPITools.Client.ClientUtils.Serialize(enumFormStringArray)); // form parameter
             }
             if (enumFormString != null)
             {
@@ -3415,6 +3429,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3600,6 +3615,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3742,6 +3758,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -3892,6 +3909,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4095,6 +4113,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -4316,6 +4335,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
index b0e5066eef8..07c4ad398c6 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs
@@ -244,6 +244,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
index 8db5fd36849..7e74501d82f 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/PetApi.cs
@@ -641,6 +641,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -843,6 +844,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1022,6 +1024,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1234,6 +1237,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1440,6 +1444,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1599,6 +1604,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1804,6 +1810,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1992,6 +1999,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -2189,6 +2197,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
index e8882b0fc2e..af79df6916f 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/StoreApi.cs
@@ -378,6 +378,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -512,6 +513,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -651,6 +653,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -792,6 +795,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/UserApi.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
index 174d0572e67..700610558ef 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Api/UserApi.cs
@@ -567,6 +567,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -709,6 +710,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -851,6 +853,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -992,6 +995,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1135,6 +1139,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1289,6 +1294,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1434,6 +1440,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -1573,6 +1580,7 @@ namespace Org.OpenAPITools.Api
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
index a0a43ea74ae..30158fdf49b 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/ApiClient.cs
@@ -385,6 +385,14 @@ namespace Org.OpenAPITools.Client
                 }
             }
 
+            if (options.HeaderParameters != null)
+            {
+                if (options.HeaderParameters.TryGetValue("Content-Type", out var contentTypes) && contentTypes.Any(header => header.Contains("multipart/form-data")))
+                {
+                    request.AlwaysMultipartFormData = true;
+                }
+            }
+
             return request;
         }