Posting multiple files to nodejs rest service from c#

I have a nodejs rest service that receives multiple files as multipart form where each element has a filename key and the actual file data.

The server part works fine (I can successfully upload files from javascript) and looks like this:

router.post("/upload_temp_resource", function(req, res){
    req.setTimeout(0);

    var form = new multiparty.Form();

    var id = String(new Date().getTime());
    var base = path.join(os.tmpdir(), id);
    var manifest = null;
    var files = [];

    form.on("part", function(part){
        if (!part.filename) return;

        var dest = path.join(base, part.name);

        files.push(part.name);

        mkdirp(path.dirname(dest), function(err){
            // ignore err if folder exists
            var stream = part.pipe(fs.createWriteStream(dest));
        });

        if (part.name=="imsmanifest.xml")
            manifest = true;
    });
    ...

The client side Javascript (which also works fine) looks like this:

var fd = new FormData();
files.forEach(function(file){
    fd.append(joinPaths(file._relativePath || "", file.name), file);
});

And fd is posted through XMLHttpRequest.

Now I need to do the same (client part) in c#. I've tried various examples found in stackoverflow but all of them generates exceptions on nodejs side (Error: stream ended unexpectedly or Error: content-type missing boundary etc)

via lviggiani