C#: HttpClient, File upload progress when uploading multiple file as MultipartFormDataContent

I have a working version of ProgressableStreamContent. Please note, I am adding headers in the constructor, this is a bug in original ProgressStreamContent that it does not add headers !!

internal class ProgressableStreamContent : HttpContent
{

    /// <summary>
    /// Lets keep buffer of 20kb
    /// </summary>
    private const int defaultBufferSize = 5*4096;

    private HttpContent content;
    private int bufferSize;
    //private bool contentConsumed;
    private Action<long,long> progress;

    public ProgressableStreamContent(HttpContent content, Action<long,long> progress) : this(content, defaultBufferSize, progress) { }

    public ProgressableStreamContent(HttpContent content, int bufferSize, Action<long,long> progress)
    {
        if (content == null)
        {
            throw new ArgumentNullException("content");
        }
        if (bufferSize <= 0)
        {
            throw new ArgumentOutOfRangeException("bufferSize");
        }

        this.content = content;
        this.bufferSize = bufferSize;
        this.progress = progress;

        foreach (var h in content.Headers) {
            this.Headers.Add(h.Key,h.Value);
        }
    }

    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {

        return Task.Run(async () =>
        {
            var buffer = new Byte[this.bufferSize];
            long size;
            TryComputeLength(out size);
            var uploaded = 0;


            using (var sinput = await content.ReadAsStreamAsync())
            {
                while (true)
                {
                    var length = sinput.Read(buffer, 0, buffer.Length);
                    if (length <= 0) break;

                    //downloader.Uploaded = uploaded += length;
                    uploaded += length;
                    progress?.Invoke(uploaded, size);

                    //System.Diagnostics.Debug.WriteLine($"Bytes sent {uploaded} of {size}");

                    stream.Write(buffer, 0, length);
                    stream.Flush();
                }
            }
            stream.Flush();
        });
    }

    protected override bool TryComputeLength(out long length)
    {
        length = content.Headers.ContentLength.GetValueOrDefault();
        return true;
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            content.Dispose();
        }
        base.Dispose(disposing);
    }

}

Also note, it expects HttpContent, not stream.

This is how you can use it.

 var progressContent = new ProgressableStreamContent (
     requestContent, 
     4096,
     (sent,total) => {
        Console.WriteLine ("Uploading {0}/{1}", sent, total);
    });