ASP.NET Web API, unexpected end of MIME multi-part stream when uploading from Flex FileReference

Question

Following the tutorial found on ASP.NET, implemented a Web API controller method for doing asynchronous file uploads that looks like this:

public Task<HttpResponseMessage> PostFormData()
{
    // Check if the request contains multipart/form-data.
    if (!Request.Content.IsMimeMultipartContent())
    {
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    }

    string root = HttpContext.Current.Server.MapPath("~/App_Data");
    var provider = new MultipartFormDataStreamProvider(root);

    // Read the form data and return an async task.
    var task = Request.Content.ReadAsMultipartAsync(provider).
        ContinueWith<HttpResponseMessage>(t =>
        {
            if (t.IsFaulted || t.IsCanceled)
            {
                Request.CreateErrorResponse(HttpStatusCode.InternalServerError, t.Exception);
            }

            return Request.CreateResponse(HttpStatusCode.OK);
        });

    return task;
}

Uploading a file via a standard multipart HTML form works perfectly. However, when another developer attempts to upload a file via multipart form constructed by Flex's FileReference class, an error is thrown:

Unexpected end of MIME multipart stream. MIME multipart message is not complete.

I have no idea if the problem lies in Web API or Flex. I've found some sort of related fixes that had no affect (Multipart form POST using ASP.Net Web API), and more recently this one ("MIME multipart stream. MIME multipart message is not complete" error on webapi upload). If the second link holds true, does anyone know if it's out in the current release of Web API available via Nuget? The discussion was in May, the most recent release from Nuget was August, so I assume this fix was deployed already, and is not the root cause of my issue.

Answer 1

I had the same problem with MVC4, but Will is correct, add a name to your input.....

<input type="file" id="fileInput" name="fileInput"/>

and all the magic is back up and working!