static void uploadFileToPhoneServer(File binaryFile) ctex {
  if (binaryFile == null || !binaryFile.isFile()) ret;
  
  S ip = first(detectGateways());
  assertNotNull(ip);
  S url = "http://" + ip + ":8888/upload";
  print("Uploading to: " + url);
  new Map<S, S> params;

  String charset = "UTF-8";
  String boundary = randomID(20); // Just generate some unique random value.
  String CRLF = "\r\n"; // Line separator required by multipart/form-data.

  URLConnection connection = new URL(url).openConnection();
  connection.setDoOutput(true);
  connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);

  OutputStream output = connection.getOutputStream();
  PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, charset), true);

  // Send normal param. (TODO: I don't think this works)
  for (S param : keys(params)) {
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"" + urlencode(param) + "\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
    // TODO: encode??
    writer.append(CRLF).append(urlencode(params.get(param))).append(CRLF);
  }

  // Send binary file.
  writer.append("--" + boundary).append(CRLF);
  writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + binaryFile.getName() + "\"").append(CRLF);
  writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
  writer.append("Content-Transfer-Encoding: binary").append(CRLF);
  writer.append(CRLF).flush();
  file2stream(binaryFile, output);
  output.flush(); // Important before continuing with writer!
  writer.append(CRLF).flush(); // CRLF is important! It indicates end of boundary.

  // End of multipart/form-data.
  writer.append("--" + boundary + "--").append(CRLF).flush();
  writer.close();
  output.close();
  
  print("Sent.");
  print();
  print(loadPage(connection, new URL(url)));
}

Began life as a copy of #1003776