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

urlConnection.set
urlConnection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
urlConnection.setDoOutput(true);

// Code zur längenberechnung:
long len = 0L;
for (String requestKey : postRequest.keySet()){
    len += ("--" + boundary).length();

    if (postRequest.get(requestKey) instanceof File){
        File requestValue = (File) postRequest.get(requestKey);
        len += ("Content-Disposition: form-data; name=\""
                + requestKey + "\"; filename=\""
                + requestValue.getName() + "\"" + CRLF
                + "Content-Type: "
                + URLConnection.guessContentTypeFromName(requestValue.getName())
                + CRLF + CRLF).length();
        len += requestValue.length();
        len += (CRLF).length();
    } else {
        if (postRequest.get(requestKey) == null)
            continue;

        len += ("Content-Disposition: form-data; name=\""
                + requestKey + "\"" + CRLF + CRLF).length();
        len += (postRequest.get(requestKey) + CRLF).length();
    }
}

len += ("--" + boundary + "--" + CRLF).length();

urlConnection.setRequestProperty("Content-Length", Long.toString(len));
System.out.println(len);
urlConnection.connect();
OutputStream os = urlConnection.getOutputStream();
PrintWriter writer = new PrintWriter(os);

for (String requestKey : postRequest.keySet()){
    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];
            CountingOutputStream cos = new CountingOutputStream((ByteArrayOutputStream) os);
            for (int length = 0; (length = input.read(buffer)) > 0;) {
                cos.write(buffer);
                lenghtUploaded += length;
                byte percentage = (byte) (lenghtUploaded / (totalLenght / 100));
                if ((percentage != lastPercentage) && (uploadObserver != null))
                    uploadObserver.onPercentageChange(lastPercentage, percentage);

                lastPercentage = percentage;
                cos.flush();
                os.flush();
                writer.flush();
                urlConnection.connect();
            }

            os.flush(); // Important! Output cannot be closed. Close of writer will close output as well.
        } finally {
            if (input != null) try { input.close(); } catch (IOException logOrIgnore) {}
        }
        writer.append(CRLF).flush(); // CRLF is important! It indicates end of binary boundary.


    } else {
        if (postRequest.get(requestKey) == null)
            continue;

        writer.append("Content-Disposition: form-data; name=\""
                + requestKey + "\"");
        writer.append(CRLF).append(CRLF);
        writer.append(postRequest.get(requestKey).toString())
                .append(CRLF).flush();
    }
}
writer.append("--" + boundary + "--").append(CRLF).flush();

os.close();

BufferedReader reader = this.getBufferOutputReader();
String line = "";

while ((line = reader.readLine()) != null) {
    page += line + "\n";
}
return page;