diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/examples/ExampleGenerator.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/examples/ExampleGenerator.java
index 6471d7f0f05..a1b973e7ea7 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/examples/ExampleGenerator.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/examples/ExampleGenerator.java
@@ -171,7 +171,7 @@ public class ExampleGenerator {
                         output.add(kv);
                     }
                 } else {
-                    kv.put(EXAMPLE, "null");
+                    kv.put(EXAMPLE, "Custom MIME type example not yet supported: " + mediaType);
                     output.add(kv);
                 }
             }
diff --git a/modules/openapi-generator/src/test/resources/3_0/csharp/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml b/modules/openapi-generator/src/test/resources/3_0/csharp/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml
index 4e4d668d055..b9e2888f551 100644
--- a/modules/openapi-generator/src/test/resources/3_0/csharp/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/csharp/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml
@@ -1069,6 +1069,10 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/ApiResponse'
+            application/vnd.openxmlformats-officedocument.spreadsheetml.sheet:
+              # Export data as Excel spreadsheet.
+              schema:
+                $ref: '#/components/schemas/ApiResponse'
       security:
         - petstore_auth:
             - 'write:pets'
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/docs/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/docs/PetApi.md
index 589980bafe6..1c51cb3d775 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/docs/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/docs/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs
index 562c0f4807f..125b0f5cb78 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Api/PetApi.cs
@@ -2033,7 +2033,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
@@ -2122,7 +2123,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/apis/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/apis/PetApi.md
index 18c0d4baf38..4187e44c57d 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/apis/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/apis/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/PetApi.cs
index c667da14abd..64cad62fdcb 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/PetApi.cs
@@ -1654,7 +1654,8 @@ namespace Org.OpenAPITools.Api
                         request.Content.Headers.Add("ContentType", contentType);
 
                     string[] accepts = new string[] { 
-                        "application/json" 
+                        "application/json", 
+                        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" 
                     };
 
                     string? accept = ClientUtils.SelectHeaderAccept(accepts);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/apis/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/apis/PetApi.md
index f651b3db0d4..8ece47af9ca 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/apis/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/apis/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Api/PetApi.cs
index 0e266cbd15a..c245714fbf1 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Api/PetApi.cs
@@ -1513,7 +1513,8 @@ namespace Org.OpenAPITools.Api
                         request.Content.Headers.Add("ContentType", contentType);
 
                     string[] accepts = new string[] { 
-                        "application/json" 
+                        "application/json", 
+                        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" 
                     };
 
                     string accept = ClientUtils.SelectHeaderAccept(accepts);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/apis/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/apis/PetApi.md
index f651b3db0d4..8ece47af9ca 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/apis/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/apis/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Api/PetApi.cs
index 6239ae756bd..82a0d311cf3 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Api/PetApi.cs
@@ -1513,7 +1513,8 @@ namespace Org.OpenAPITools.Api
                         request.Content.Headers.Add("ContentType", contentType);
 
                     string[] accepts = new string[] { 
-                        "application/json" 
+                        "application/json", 
+                        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" 
                     };
 
                     string accept = ClientUtils.SelectHeaderAccept(accepts);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/docs/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/docs/PetApi.md
index 9f3f7aa9f22..9db1797db99 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/docs/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/docs/PetApi.md
@@ -880,7 +880,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Api/PetApi.cs
index a4ab1260df9..6f8299aee6c 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Api/PetApi.cs
@@ -1871,7 +1871,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
@@ -1945,7 +1946,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
 
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/docs/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/docs/PetApi.md
index 589980bafe6..1c51cb3d775 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/docs/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/docs/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Api/PetApi.cs
index 562c0f4807f..125b0f5cb78 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Api/PetApi.cs
@@ -2033,7 +2033,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
@@ -2122,7 +2123,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/docs/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/docs/PetApi.md
index 589980bafe6..1c51cb3d775 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/docs/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/docs/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Api/PetApi.cs
index 562c0f4807f..125b0f5cb78 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Api/PetApi.cs
@@ -2033,7 +2033,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
@@ -2122,7 +2123,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient/docs/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClient/docs/PetApi.md
index 589980bafe6..1c51cb3d775 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient/docs/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient/docs/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Api/PetApi.cs
index 562c0f4807f..125b0f5cb78 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Api/PetApi.cs
@@ -2033,7 +2033,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
@@ -2122,7 +2123,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/docs/PetApi.md b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/docs/PetApi.md
index 589980bafe6..1c51cb3d775 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/docs/PetApi.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/docs/PetApi.md
@@ -844,7 +844,7 @@ catch (ApiException e)
 ### HTTP request headers
 
  - **Content-Type**: multipart/form-data
- - **Accept**: application/json
+ - **Accept**: application/json, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
 
 
 ### HTTP response details
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Api/PetApi.cs b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Api/PetApi.cs
index 562c0f4807f..125b0f5cb78 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Api/PetApi.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Api/PetApi.cs
@@ -2033,7 +2033,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
@@ -2122,7 +2123,8 @@ namespace Org.OpenAPITools.Api
 
             // to determine the Accept header
             string[] _accepts = new string[] {
-                "application/json"
+                "application/json",
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
             };
 
             var localVarContentType = Org.OpenAPITools.Client.ClientUtils.SelectHeaderContentType(_contentTypes);