Unity3d : 500 internal server error : while uploading file with WWW and wwwform

See more:

C#

ASP.NET

WebAPI

Unity3D

I am trying to upload a file to the server using www and wwwForm from the client Side using this code :

WWW localFile = new WWW("file://"+ FilePath);
yield return localFile;

WWWForm postForm = new WWWForm();
postForm.AddField("frameCount", Time.frameCount.ToString());
postForm.AddBinaryData("file", localFile.bytes, "filename.obj");

WWW upload = new WWW(uploadUrl, postForm);
yield return upload;

if (upload.error == null)
{
    Debug.Log(upload.text);
}
else
{
    Debug.Log("Error during upload: " + upload.error);
}

Calling the ASP.net Service Upload File :

public async Task<HttpResponseMessage> UploadFileAsync()
        {
            try
            {
                var provider = new MultipartMemoryStreamProvider();
                await Request.Content.ReadAsMultipartAsync(provider);
                foreach (var file in provider.Contents)
                {
                    var dataStream = await file.ReadAsStreamAsync();
                    // use the data stream to persist the data to the server (file system etc)
                    Stream fileStream = File.Create("D:\\filename.obj");
                    dataStream.CopyTo(fileStream);
                    fileStream.Close();
                    dataStream.Close();
                    var response = Request.CreateResponse(HttpStatusCode.OK);
                    response.Content = new StringContent("Successful upload", Encoding.UTF8, "text/plain");
                    response.Content.Headers.ContentType = new MediaTypeWithQualityHeaderValue(@"text/html");
                    return response;
                }
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, "No File Uploaded");
            }
            catch (Exception e)
            {
                return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e.Message);
            }
        }

and I am getting ( 500 Internal Server Error)?
the service was running Perfectly, but something wrong Happen, I get 500 Internal Server Error
So what is the Wrong with That?
Thanks in Advance 

What I have tried:

The Service was running Perfectly, i tried to increase the maximumFileSize ,
also increasing the request length didn't solve the Issue