Loaded "multipart / form-data" in ASP.NET Web API activity that cannot be read in no buffer mode

I am creating an ASP.NET Web API endpoint that accepts "multipart / form-data" requests. I implemented it as described in this article using .NET Framework 4.5 and Web API 2.1. A simplified version of the action method I created looks like this:

public async Task<HttpResponseMessage> PostFile()
{
    if (!Request.Content.IsMimeMultipartContent()) throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    var rootPath = System.Configuration.ConfigurationManager.AppSettings["StorageLocation"].ToString();
    var provider = new MultipartFormDataStreamProvider(rootPath);
    var response = Request.CreateResponse(HttpStatusCode.OK);
    try
    {
        await Request.Content.ReadAsMultipartAsync(provider);
        // Imagine awesome logic here, unicorns and rainbows! Instead of that, we do the following:
        response.Content = new StringContent("You uploaded " + provider.FileData.Count.ToString() + " files.");
    }
    catch (Exception e) { throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e)); }
    return response;
}

      

        
        
        
      

    

Since the uploaded files can be very large (up to 2GiB), I want my requests not to be buffered by ASP.NET, thus avoiding high memory usage. To implement this, I told the Web API to stream incoming requests instead of buffering them as described in this article . A custom WebHostBufferPolicySelector looks something like this:

public class CustomWebHostBufferPolicySelector : WebHostBufferPolicySelector
{
    public override bool UseBufferedInputStream(object hostContext)
    {
        System.Web.HttpContextBase contextBase = hostContext as System.Web.HttpContextBase;
        if (contextBase != null && contextBase.Request.ContentType != null && contextBase.Request.ContentType.Contains("multipart")) return false;
        else return base.UseBufferedInputStream(hostContext);
    }

    public override bool UseBufferedOutputStream(System.Net.Http.HttpResponseMessage response)
    {
        return base.UseBufferedOutputStream(response);
    }
}

      

        
        
        
      

    

I am loading this guy into Global.asax when starting the application, for example:

protected void Application_Start(object sender, EventArgs e)
{
    // Here, other stuff got did.
    GlobalConfiguration.Configuration.Services.Replace(typeof(IHostBufferPolicySelector), new CustomWebHostBufferPolicySelector());
}

      

        
        
        
      

    

Okay, the board is set up, lets you move the pieces. If I don't use my CustomWebHostBufferPolicySelector everything works fine. However, when it is used, I get the following exception:

Message: "An error has occurred."
ExceptionMessage: "Error reading MIME multipart body part."
ExceptionType: "System.IO.IOException"
StackTrace: " at System.Net.Http.HttpContentMultipartExtensions.<ReadAsMultipartAsync>d__0`1.MoveNext()\ \ --- End of stack trace from previous location where exception was thrown ---\ \ at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\ \ at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\ \ at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\ \ at ..."

      

        
        
        
      

    

With the following internal exception:

Message: "An error has occurred."
ExceptionMessage: "Unable to read the entity body in Bufferless mode. The request stream has already been buffered."
ExceptionType: "System.InvalidOperationException"
StackTrace: " at System.Web.Http.WebHost.HttpControllerHandler.<>c__DisplayClass13.<GetStreamContent>b__10()\ \ at System.Web.Http.WebHost.HttpControllerHandler.LazyStreamContent.get_StreamContent()\ \ at System.Web.Http.WebHost.HttpControllerHandler.LazyStreamContent.CreateContentReadStreamAsync()\ \ at System.Net.Http.HttpContent.ReadAsStreamAsync()\ \ at System.Net.Http.HttpContentMultipartExtensions.<ReadAsMultipartAsync>d__0`1.MoveNext()"

      

        
        
        
      

    

It looks like the request is still buffered in some way, something else. Is there a place in the ASP.NET pipeline that I should be looking for? Or even IIS? What are the other places in this request lifecycle where it can be buffered and how can I control them?