An updated model binder that supports async and non-async file uploads.

HomeController.cs

namespace AsyncUpload.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            ViewBag.Message = "Welcome to ASP.NET MVC!";

            return View();
        }

        public ActionResult About()
        {
            return View();
        }

        [HttpPost]
        public ActionResult Upload(HttpPostedFileBase file)
        {
            // Use the standard HttpPostedFileBase

            return View();
        }
    }
}

HttpPostedFileBaseModelBinder.cs

namespace AsyncUpload.Models
{
    public class HttpPostedFileBaseModelBinder : IModelBinder
    {
        public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            var request = controllerContext.RequestContext.HttpContext.Request;
            if (request.Files.Count > 0)
            {
                var file = request.Files[bindingContext.ModelName];
                if (file != null && file.ContentLength > 0 && !string.IsNullOrEmpty(file.FileName))
                    return file;

                return null;
            }

            if (request.ContentLength > 0)
            {
                string filename = request.Headers["X-File-Name"];
                if (string.IsNullOrEmpty(filename))
                    return null;

                string fileType = request.Headers["X-File-Type"];
                byte[] fileContent = new byte[request.ContentLength];
                
                request.InputStream.Read(fileContent, 0, request.ContentLength);
                
                return new AsyncHttpPostedFile(filename, fileType, fileContent);
            }
            return null;
        }

        class AsyncHttpPostedFile : HttpPostedFileBase
        {
            private readonly string _filename;
            private readonly string _contentType;
            private readonly int _contentLength;
            private readonly byte[] _data;

            public AsyncHttpPostedFile(string filename, string contentType, byte[] data)
            {
                _filename = filename;
                _contentType = contentType;
                _contentLength = data.Length;
                _data = data;
            }

            public override string FileName { get { return _filename; } }
            public override string ContentType { get { return _contentType; } }
            public override int ContentLength { get { return _contentLength; } }
            public override Stream InputStream { get { return new MemoryStream(_data); } }
        }
    }
}