[Kotlin][client] fix file upload (#5548)

* [kotlin] fix file upload

* [kotlin] fix file upload

* [kotlin] fix file upload

* [kotlin][client] fix jackson integration

* [kotlin] fix file upload

* [kotlin] fix file upload

modules/openapi-generator/src/main/resources/kotlin-client/api.mustache

@@ -32,7 +32,7 @@ import {{packageName}}.infrastructure.toMultiValue
     @Suppress("UNCHECKED_CAST"){{/returnType}}
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun {{operationId}}({{#allParams}}{{{paramName}}}: {{{dataType}}}{{^required}}?{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}}) : {{#returnType}}{{{returnType}}}{{#nullableReturnType}}?{{/nullableReturnType}}{{/returnType}}{{^returnType}}Unit{{/returnType}} {
-        val localVariableBody: kotlin.Any? = {{#hasBodyParam}}{{#bodyParams}}{{{paramName}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}null{{/hasFormParams}}{{#hasFormParams}}mapOf({{#formParams}}"{{{baseName}}}" to "${{{paramName}}}"{{#hasMore}}, {{/hasMore}}{{/formParams}}){{/hasFormParams}}{{/hasBodyParam}}
+        val localVariableBody: kotlin.Any? = {{#hasBodyParam}}{{#bodyParams}}{{{paramName}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}null{{/hasFormParams}}{{#hasFormParams}}mapOf({{#formParams}}"{{{baseName}}}" to {{{paramName}}}{{#hasMore}}, {{/hasMore}}{{/formParams}}){{/hasFormParams}}{{/hasBodyParam}}
         val localVariableQuery: MultiValueMap = {{^hasQueryParams}}mutableMapOf()
 {{/hasQueryParams}}{{#hasQueryParams}}mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
@@ -55,7 +55,7 @@ import {{packageName}}.infrastructure.toMultiValue
                 {{/queryParams}}
             }
             {{/hasQueryParams}}
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf({{#hasFormParams}}"Content-Type" to {{^consumes}}"multipart/form-data"{{/consumes}}{{#consumes.0}}"{{MediaType}}"{{/consumes.0}}{{/hasFormParams}}{{^hasHeaderParams}}){{/hasHeaderParams}}{{#hasHeaderParams}}{{#hasFormParams}}, {{/hasFormParams}}{{#headerParams}}"{{baseName}}" to {{#isContainer}}{{{paramName}}}.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}{{{paramName}}}.toString(){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/headerParams}}){{/hasHeaderParams}}
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf({{#hasFormParams}}"Content-Type" to {{^consumes}}"multipart/form-data"{{/consumes}}{{#consumes.0}}"{{{mediaType}}}"{{/consumes.0}}{{/hasFormParams}}{{^hasHeaderParams}}){{/hasHeaderParams}}{{#hasHeaderParams}}{{#hasFormParams}}, {{/hasFormParams}}{{#headerParams}}"{{baseName}}" to {{#isContainer}}{{{paramName}}}.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}{{{paramName}}}.toString(){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/headerParams}}){{/hasHeaderParams}}
         val localVariableConfig = RequestConfig(
             RequestMethod.{{httpMethod}},
             "{{path}}"{{#pathParams}}.replace("{"+"{{baseName}}"+"}", "${{{paramName}}}"){{/pathParams}},

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/infrastructure/ApiClient.kt.mustache

@@ -22,7 +22,25 @@ import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 {{/jvm-okhttp4}}
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+{{^threetenbp}}
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
+{{/threetenbp}}
+{{#threetenbp}}
+import org.threeten.bp.LocalDate
+import org.threeten.bp.LocalDateTime
+import org.threeten.bp.LocalTime
+import org.threeten.bp.OffsetDateTime
+import org.threeten.bp.OffsetTime
+{{/threetenbp}}
 
 {{#nonPublicApi}}internal {{/nonPublicApi}}open class ApiClient(val baseUrl: String) {
     {{#nonPublicApi}}internal {{/nonPublicApi}}companion object {
@@ -49,6 +67,17 @@ import java.io.File
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             {{#jvm-okhttp3}}
@@ -61,12 +90,50 @@ import java.io.File
                 mediaType.toMediaTypeOrNull()
             )
             {{/jvm-okhttp4}}
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.{{#jvm-okhttp3}}of{{/jvm-okhttp3}}{{#jvm-okhttp4}}headersOf{{/jvm-okhttp4}}(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                {{#jvm-okhttp3}}
+                                val fileMediaType = MediaType.parse(guessContentTypeFromFile(value))
+                                addPart(partHeaders, RequestBody.create(fileMediaType, value))
+                                {{/jvm-okhttp3}}
+                                {{#jvm-okhttp4}}
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                                {{/jvm-okhttp4}}
+                            } else {
+                                val partHeaders = Headers.{{#jvm-okhttp3}}of{{/jvm-okhttp3}}{{#jvm-okhttp4}}headersOf{{/jvm-okhttp4}}(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    {{#jvm-okhttp3}}
+                                    RequestBody.create(null, parameterToString(value))
+                                    {{/jvm-okhttp3}}
+                                    {{#jvm-okhttp4}}
+                                    parameterToString(value).toRequestBody(null)
+                                    {{/jvm-okhttp4}}
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -79,7 +146,7 @@ import java.io.File
                 MediaType.parse(mediaType), Serializer.gson.toJson(content, T::class.java)
                 {{/gson}}
                 {{#jackson}}
-                MediaType.parse(mediaType), Serializer.jackson.toJson(content, T::class.java)
+                MediaType.parse(mediaType), Serializer.jacksonObjectMapper.writeValueAsString(content)
                 {{/jackson}}
             )
             {{/jvm-okhttp3}}
@@ -254,7 +321,26 @@ import java.io.File
         }
     }
 
-    {{^jackson}}
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         {{#toJson}}
         /*
@@ -269,10 +355,12 @@ import java.io.File
         {{#gson}}
         return Serializer.gson.toJson(value, T::class.java).replace("\"", "")
         {{/gson}}
+        {{#jackson}}
+        return Serializer.jacksonObjectMapper.writeValueAsString(value).replace("\"", "")
+        {{/jackson}}
         {{/toJson}}
         {{^toJson}}
         return value.toString()
         {{/toJson}}
     }
-    {{/jackson}}
 }

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,4 +219,33 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
+    protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
+        /*
+        .replace("\"", "") converts the json object string to an actual string for the query parameter.
+        The moshi or gson adapter allows a more generic solution instead of trying to use a native
+        formatter. It also easily allows to provide a simple way to define a custom date format pattern
+        inside a gson/moshi adapter.
+        */
+        return Serializer.jacksonObjectMapper.writeValueAsString(value).replace("\"", "")
+    }
 }

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -293,9 +293,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -336,9 +336,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         return value.toString()
     }

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ internal class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2")
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ internal class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2")
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 internal open class ApiClient(val baseUrl: String) {
     internal companion object {
@@ -37,17 +46,55 @@ internal open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ internal open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse? {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -8,7 +8,16 @@ import okhttp3.FormBody
 import okhttp3.HttpUrl
 import okhttp3.ResponseBody
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -35,17 +44,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> RequestBody.create(
                 MediaType.parse(mediaType), content
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.of(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = MediaType.parse(guessContentTypeFromFile(value))
+                                addPart(partHeaders, RequestBody.create(fileMediaType, value))
+                            } else {
+                                val partHeaders = Headers.of(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    RequestBody.create(null, parameterToString(value))
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -170,6 +217,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import org.threeten.bp.LocalDate
+import org.threeten.bp.LocalDateTime
+import org.threeten.bp.LocalTime
+import org.threeten.bp.OffsetDateTime
+import org.threeten.bp.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -291,9 +291,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     */
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
-        val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
+        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
@@ -334,9 +334,9 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     @Throws(UnsupportedOperationException::class, ClientException::class, ServerException::class)
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
+        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "")
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
         val localVariableConfig = RequestConfig(
             RequestMethod.POST,
             "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,7 +10,16 @@ import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.ResponseBody
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
+import okhttp3.Headers
+import okhttp3.MultipartBody
 import java.io.File
+import java.net.URLConnection
+import java.util.Date
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.LocalTime
+import java.time.OffsetDateTime
+import java.time.OffsetTime
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -37,17 +46,55 @@ open class ApiClient(val baseUrl: String) {
         val builder: OkHttpClient.Builder = OkHttpClient.Builder()
     }
 
+    /**
+     * Guess Content-Type header from the given file (defaults to "application/octet-stream").
+     *
+     * @param file The given file
+     * @return The guessed Content-Type
+     */
+    protected fun guessContentTypeFromFile(file: File): String {
+        val contentType = URLConnection.guessContentTypeFromName(file.name)
+        return contentType ?: "application/octet-stream"
+    }
+
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             content is File -> content.asRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
-            mediaType == FormDataMediaType || mediaType == FormUrlEncMediaType -> {
+            mediaType == FormDataMediaType -> {
+                MultipartBody.Builder()
+                    .setType(MultipartBody.FORM)
+                    .apply {
+                        // content's type *must* be Map<String, Any?>
+                        @Suppress("UNCHECKED_CAST")
+                        (content as Map<String, Any?>).forEach { (key, value) ->
+                            if (value is File) {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\"; filename=\"${value.name}\""
+                                )
+                                val fileMediaType = guessContentTypeFromFile(value).toMediaTypeOrNull()
+                                addPart(partHeaders, value.asRequestBody(fileMediaType))
+                            } else {
+                                val partHeaders = Headers.headersOf(
+                                    "Content-Disposition",
+                                    "form-data; name=\"$key\""
+                                )
+                                addPart(
+                                    partHeaders,
+                                    parameterToString(value).toRequestBody(null)
+                                )
+                            }
+                        }
+                    }.build()
+            }
+            mediaType == FormUrlEncMediaType -> {
                 FormBody.Builder().apply {
-                    // content's type *must* be Map<String, Any>
+                    // content's type *must* be Map<String, Any?>
                     @Suppress("UNCHECKED_CAST")
-                    (content as Map<String,String>).forEach { (key, value) ->
-                        add(key, value)
+                    (content as Map<String, Any?>).forEach { (key, value) ->
+                        add(key, parameterToString(value))
                     }
                 }.build()
             }
@@ -172,6 +219,26 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
+    protected fun parameterToString(value: Any?): String {
+        when (value) {
+            null -> {
+                return ""
+            }
+            is Array<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is Iterable<*> -> {
+                return toMultiValue(value, "csv").toString()
+            }
+            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
+                return parseDateToQueryString<Any>(value)
+            }
+            else -> {
+                return value.toString()
+            }
+        }
+    }
+
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {
         /*
         .replace("\"", "") converts the json object string to an actual string for the query parameter.