Fix the custom boundary format

Author: alexeyzimarev

src/RestSharp/Request/RequestContent.cs

@@ -44,7 +44,13 @@ public HttpContent BuildContent() {
         var postParameters = _request.Parameters.GetContentParameters(_request.Method);
         AddBody(!postParameters.IsEmpty());
         AddPostParameters(postParameters);
+
+        if (Content is MultipartFormDataContent && _request.FormatMultipartContentType != null) {
+            ReplaceHeader(ContentType, GetContentTypeHeader(Content.Headers.ContentType!.MediaType!));
+        }
+
         AddHeaders();
+
         return Content!;
     }
 
@@ -180,16 +186,25 @@ void AddHeader(Parameter parameter) {
                 _           => parameterStringValue
             };
             var pName = Ensure.NotNull(parameter.Name, nameof(parameter.Name));
-            Content!.Headers.Remove(pName);
-            Content!.Headers.TryAddWithoutValidation(pName, value);
+            ReplaceHeader(pName, value);
         }
     }
 
-    string GetContentTypeHeader(string contentType)
-        => Content is MultipartFormDataContent mpContent
-            ? _request.FormatMultipartContentType(contentType, mpContent.GetFormBoundary())
+    void ReplaceHeader(string name, string? value) {
+        Content!.Headers.Remove(name);
+        Content!.Headers.TryAddWithoutValidation(name, value);
+    }
+
+    static readonly FormatContentTypeHeader DefaultContentTypeHeader = (contentType, boundary) => $"{contentType}; boundary=\"{boundary}\"";
+
+    string GetContentTypeHeader(string contentType) {
+        return Content is MultipartFormDataContent mpContent
+            ? ContentTypeValue()(contentType, mpContent.GetFormBoundary())
             : contentType;
 
+        FormatContentTypeHeader ContentTypeValue() => _request.FormatMultipartContentType ?? DefaultContentTypeHeader;
+    }
+
     public void Dispose() {
         _streams.ForEach(x => x.Dispose());
         Content?.Dispose();

src/RestSharp/Request/RestRequest.cs

@@ -150,7 +150,7 @@ public RestRequest(Uri resource, Method method = Method.Get)
     /// <summary>
     /// Function that formats the content type header for multipart form fata
     /// </summary>
-    public FormatContentTypeHeader FormatMultipartContentType { get; set; } = (contentType, boundary) => $"{contentType}; boundary=\"{boundary}\"";
+    public FormatContentTypeHeader? FormatMultipartContentType { get; set; }
 
     /// <summary>
     /// Set this to write response to Stream rather than reading into memory.

test/RestSharp.Tests.Integrated/MultipartFormDataTests.cs

@@ -177,6 +177,30 @@ public async Task MultipartFormData_WithParameterAndFile_Async() {
         response.Content.Should().Be(expectedFileAndBodyRequestContent);
     }
 
+    [Fact]
+    public async Task MultipartFormDataWithBoundaryOverride() {
+        var request = new RestRequest("/", Method.Post) {
+            AlwaysMultipartFormData = true,
+            FormatMultipartContentType = (ct, b) => $"{ct}; boundary=--------{b}"
+        };
+
+        AddParameters(request);
+
+        HttpContent content  = null;
+        var      boundary = "";
+
+        request.OnBeforeRequest = http => {
+            content  = http.Content;
+            boundary = ((MultipartFormDataContent)http.Content)!.GetFormBoundary();
+            return default;
+        };
+        
+        await _client.ExecuteAsync(request);
+
+        var contentType = content.Headers.ContentType!.ToString();
+        contentType.Should().Be($"multipart/form-data; boundary=--------{boundary}");
+    }
+
     [Fact]
     public async Task MultipartFormDataAsync() {
         var request = new RestRequest("/", Method.Post) { AlwaysMultipartFormData = true };