Quote multipart form parameter names by default (RFC 7578) (#2357)

* Quote multipart form parameter names by default (RFC 7578)

Change MultipartFormQuoteParameters default from false to true so that
parameter names in Content-Disposition headers are quoted consistently
with file parameter names, as required by RFC 7578.

Fixes #2271

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Apply parameter name quoting to body parts in multipart form data

Extend MultipartFormQuoteParameters to also cover BodyParameter parts
added via AddBody, ensuring consistent Content-Disposition name quoting
across all multipart form parts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

src/RestSharp/Request/RequestContent.cs

@@ -143,7 +143,7 @@ void AddBody(bool hasPostParameters, BodyParameter bodyParameter) {
             if (name.IsEmpty())
                 mpContent.Add(bodyContent);
             else
-                mpContent.Add(bodyContent, name);
+                mpContent.Add(bodyContent, request.MultipartFormQuoteParameters ? $"\"{name}\"" : name);
             Content = mpContent;
         }
         else {

src/RestSharp/Request/RestRequest.cs

@@ -87,10 +87,9 @@ public RestRequest(Uri resource, Method method = Method.Get)
 
     /// <summary>
     /// When set to true, parameter values in a multipart form data requests will be enclosed in
-    /// quotation marks. Default is false. Enable it if the remote endpoint requires parameters
-    /// to be in quotes (for example, FreshDesk API). 
+    /// quotation marks. Default is true, as per RFC 7578.
     /// </summary>
-    public bool MultipartFormQuoteParameters { get; set; }
+    public bool MultipartFormQuoteParameters { get; set; } = true;
 
     /// <summary>
     /// When set to true, the form boundary part of the content type will be enclosed in

test/RestSharp.Tests.Integrated/MultipartFormDataTests.cs

@@ -31,7 +31,7 @@ public void Dispose() {
     const string ContentDispositionString = $"{KnownHeaders.ContentDisposition}: form-data;";
 
     const string Expected =
-        $"--{{0}}{LineBreak}{ContentTypeString}{LineBreak}{ContentDispositionString} name=foo{LineBreak}{LineBreak}bar{LineBreak}" +
+        $"--{{0}}{LineBreak}{ContentTypeString}{LineBreak}{ContentDispositionString} name=\"foo\"{LineBreak}{LineBreak}bar{LineBreak}" +
         $"--{{0}}{LineBreak}{ContentTypeString}{LineBreak}{ContentDispositionString} name=\"a name with spaces\"{LineBreak}{LineBreak}somedata{LineBreak}" +
         $"--{{0}}--{LineBreak}";
 
@@ -41,7 +41,7 @@ public void Dispose() {
         $"{LineBreak}{KnownHeaders.ContentDisposition}: form-data; name=\"fileName\"; filename=\"TestFile.txt\"" +
         $"{LineBreak}{LineBreak}This is a test file for RestSharp.{LineBreak}" +
         $"--{{0}}{LineBreak}{KnownHeaders.ContentType}: application/json; {CharsetString}" +
-        $"{LineBreak}{KnownHeaders.ContentDisposition}: form-data; name=controlName" +
+        $"{LineBreak}{KnownHeaders.ContentDisposition}: form-data; name=\"controlName\"" +
         $"{LineBreak}{LineBreak}test{LineBreak}" +
         $"--{{0}}--{LineBreak}";
 
@@ -199,7 +199,7 @@ public async Task MultipartFormData_Without_File_Creates_A_Valid_RequestBody() {
 
         var expectedBody = new[] {
             ContentTypeString,
-            $"{ContentDispositionString} name={multipartName}",
+            $"{ContentDispositionString} name=\"{multipartName}\"",
             bodyData
         };
 
@@ -228,7 +228,7 @@ public async Task PostParameter_contentType_in_multipart_form() {
 
         var actual = capturer.Body!.Replace("\n", string.Empty).Split('\r');
         actual[1].Should().Be("Content-Type: application/json; charset=utf-8");
-        actual[2].Should().Be($"Content-Disposition: form-data; name={parameterName}");
+        actual[2].Should().Be($"Content-Disposition: form-data; name=\"{parameterName}\"");
         actual[4].Should().Be(parameterValue);
     }
 }