URLConnection urlConnection = new URL("http://meineSeite.de/upload.php").openConnection();

((HttpURLConnection) urlConnection).setRequestMethod("POST");
String boundary = Long.toHexString(System.currentTimeMillis()); // Just generate some unique random value.
String CRLF = "\r\n"; // Line separator required by multipart/form-data.

urlConnection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
urlConnection.setDoOutput(true);
urlConnection.setDoInput(true);
urlConnection.setUseCaches(false);
urlConnection.setDefaultUseCaches(false);
OutputStream os = urlConnection.getOutputStream();

PrintWriter writer = new PrintWriter(os);

writer.append("--" + boundary).append(CRLF);
if (postRequest.get(requestKey) instanceof File) {
    File requestValue = (File) postRequest.get(requestKey);
    writer.append("Content-Disposition: form-data; name=\""
            + requestKey + "\"; filename=\""
            + requestValue.getName() + "\"").append(CRLF);
    writer.append("Content-Type: "
            + URLConnection.guessContentTypeFromName(requestValue.getName())).append(CRLF);
    writer.append(CRLF).flush();
    InputStream input = null;
    try {
        final long totalLenght = requestValue.length();
        long lenghtUploaded = 0L;
        input = new FileInputStream(requestValue);
        byte lastPercentage = -1;
        byte[] buffer = new byte[100];
        for (int length = 0; (length = input.read(buffer)) > 0;) {
            os.write(buffer, 0, length);
            lenghtUploaded = lenghtUploaded + length;
            byte percentage = (byte) (lenghtUploaded / (totalLenght / 100));
            if ((percentage != lastPercentage) && (uploadObserver != null))
                uploadObserver.onPercentageChange(lastPercentage, percentage);


            lastPercentage = percentage;
            os.flush();
            writer.flush();
        }

        os.flush();
    } finally {
        if (input != null) try { input.close(); } catch (IOException logOrIgnore) {}
    }
    writer.append(CRLF).flush(); // CRLF is important! It indicates end of binary boundary.
}


writer.append("--" + boundary + "--").append(CRLF).flush();
os.close();