async public Task<HttpResponseMessage> UploadImage(string url, byte[] ImageData)
{
    var requestContent = new MultipartFormDataContent();
    //    here you can specify boundary if you need---^
    var imageContent = new ByteArrayContent(ImageData);
    imageContent.Headers.ContentType = MediaTypeHeaderValue.Parse("image/jpeg");

    requestContent.Add(imageContent, "image", "image.jpg");

    return await client.PostAsync(url, requestContent);
}