Properly handle multipart/form-data content type without files

Fixes #9902

Author: cipold

modules/swagger-codegen/src/main/resources/cpprest/apiclient-source.mustache

@@ -140,7 +140,20 @@ pplx::task<web::http::http_response> ApiClient::callApi(
                 if (!formParams.empty())
                 {
                     request.set_body(body_data);
-		}
+                }
+            }
+            else if (contentType == utility::conversions::to_string_t("multipart/form-data"))
+            {
+                MultipartFormData uploadData;
+                for (auto& kvp : formParams)
+                {
+                    uploadData.add(ModelBase::toHttpContent(kvp.first, kvp.second));
+                }
+                std::stringstream data;
+                uploadData.writeTo(data);
+                auto bodyString = data.str();
+                auto length = bodyString.size();
+                request.set_body(concurrency::streams::bytestream::open_istream(std::move(bodyString)), length, utility::conversions::to_string_t("multipart/form-data; boundary=") + uploadData.getBoundary());
             }
             else
             {