Windows Phone 8 で HTTP POST マルチパート/フォームデータを使用してファイルと文字列データをアップロードする方法

async void MainPage_Loaded(object sender, RoutedEventArgs e)
{
    var file = await Windows.ApplicationModel.Package.Current.InstalledLocation.GetFileAsync(DBNAME);
    byte[] fileBytes = null;
    using (var stream = await file.OpenReadAsync())
    {
        fileBytes = new byte[stream.Size];
        using (var reader = new DataReader(stream))
        {
            await reader.LoadAsync((uint)stream.Size);
            reader.ReadBytes(fileBytes);
        }
    }

    HttpPost(fileBytes);
}

private async void HttpPost(byte[] fileBytes)
{
    HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create("http://www.myserver.com/upload.php");
    httpWebRequest.ContentType = "multipart/form-data";
    httpWebRequest.Method = "POST";
    using (var formBody = new HttpFormBody())
    {
        formBody.AddString("userid", HttpUtility.UrlEncode("SOME_ID"));
        httpWebRequest.AddString(formBody.CreateFormField());
    }

    using (var dataStream = await httpWebRequest.GetRequestStreamAsync())
    {
        dataStream.Write(fileBytes, 0, fileBytes.Length);
        dataStream.Close();
    }

    var asyncResult = httpWebRequest.BeginGetResponse(new AsyncCallback(GetResponseCallback), httpWebRequest);
}