fix: removed serialization of FormParameters for multipart/form-data

Author: lewishazell

modules/openapi-generator/src/main/resources/csharp/api.mustache

@@ -290,6 +290,7 @@ namespace {{packageName}}.{{apiPackage}}
             };
 
             var localVarContentType = {{packageName}}.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
+            var localVarMultipartFormData = localVarContentType == "multipart/form-data";
             if (localVarContentType != null)
             {
                 localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);
@@ -375,7 +376,7 @@ namespace {{packageName}}.{{apiPackage}}
             {{/isArray}}
             {{/isFile}}
             {{^isFile}}
-            localVarRequestOptions.FormParameters.Add("{{baseName}}", {{packageName}}.Client.ClientUtils.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}})); // form parameter
+            localVarRequestOptions.FormParameters.Add("{{baseName}}", {{#isPrimitiveType}}{{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}){{/isPrimitiveType}}{{^isPrimitiveType}}localVarMultipartFormData ? {{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}) : {{packageName}}.Client.ClientUtils.Serialize({{paramName}}){{/isPrimitiveType}}); // form parameter
             {{/isFile}}
             {{/required}}
             {{^required}}
@@ -397,7 +398,7 @@ namespace {{packageName}}.{{apiPackage}}
                 {{/isArray}}
                 {{/isFile}}
                 {{^isFile}}
-                localVarRequestOptions.FormParameters.Add("{{baseName}}", {{packageName}}.Client.ClientUtils.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}})); // form parameter
+                localVarRequestOptions.FormParameters.Add("{{baseName}}", {{#isPrimitiveType}}{{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}){{/isPrimitiveType}}{{^isPrimitiveType}}localVarMultipartFormData ? {{packageName}}.Client.ClientUtils.ParameterToString({{paramName}}) : {{packageName}}.Client.ClientUtils.Serialize({{paramName}}){{/isPrimitiveType}}); // form parameter
                 {{/isFile}}
             }
             {{/required}}