public async 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);

        try
        {
            // Read the form data.
            await Request.Content.ReadAsMultipartAsync(provider);

            // This illustrates how to get the file names.
            foreach (MultipartFileData file in provider.FileData)
            {
                Debug.WriteLine(file.Headers.ContentDisposition.FileName);
                Debug.WriteLine("Server file path: " + file.LocalFileName);
                Debug.WriteLine("Content type: " + file.Headers.ContentType);
                Debug.WriteLine("This wont work: " + file.Headers.ContentLength);
            }
            return Request.CreateResponse(HttpStatusCode.OK, "OK");
        }
        catch (System.Exception e)
        {
            return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e);
        }
    }

var fileSize = new FileInfo(file.LocalFileName).Length;

Debug.WriteLine("This wont work: " + file.Headers.ContentLength);