Node.js HTTP client streaming multipart upload

multipart.js

function multipart_upload_stream(url, parts, callback) {
    // TODO: better GUID
    var boundary = Math.random().toString().slice(2);

    var len = 0;
    parts = parts.map(function(part){
        part = make_part(part);
        len += part.length;
        return part;
    });

    len += (4 + boundary.length);

    var headers = {
        'Content-Type': 'multipart/form-data; boundary=' + boundary,
        'Content-Length': len
    };

    var opt = URL.parse(url);
    opt.method = 'POST';
    opt.headers = headers;
    var req = http.request(opt, function(res){});

    var $end = req.end;
    req.end = function(data){
        if(data) req.write(data);
        req.write('\r\n--' + boundary + '--');

        $end.call(req);
    }

    parts.forEach(function(part) {
        req.write(part.data);
    });

    callback(null, req);

    // TODO: filename encoding
    function make_part(p){
        var part = '--' + boundary + '\r\n'
            + 'Content-Disposition: form-data; name=' + '"' + p.name + '"'
            + (     p.filename ? ('; filename=' + '"' + p.filename + '"') : '')
            + '\r\n'
            + ( p.type ? ('Content-Type:' + p.type + '\r\n') : '' )
            + '\r\n'
            + ( p.body ? (p.body + '\r\n') : '');

        part = new Buffer(part);

        var len = 0;

        if(p.body && p.length) throw "you don't want to do that.";

        if(p.length) {
            len += p.length + part.length;
        } else if(p.body)
            len += part.length;

        return {
            length: len,
            data: part,
            stream: p.stream
        }
    }
}