How to place multi-part / image form with string parameter in android

I have an API that has a payload body of type String. But it has both jsonand image(multi-party system / form) as part of the payload body. Something like that:

json={jsonbody} image=@images/testing.png

This is what I am doing now.

public static String uploadImageQuery(Context context, String urlString, String method,
                                      JSONObject jsonObject, Bitmap largeImageBitmap,
                                      Dialog dialog) throws IOException {

    String responseString = null;
    HttpURLConnection conn = null;
    URL url = new URL(urlString);

    conn = (HttpURLConnection) url.openConnection();

    Log.d(TAG, "Uploading largeImageBitmap ..");

    conn.setConnectTimeout((int) Constants.THREE_MINUTES);
    conn.setDoOutput(true);
    conn.setDoInput(true);
    conn.setUseCaches(false);
    conn.setRequestMethod(method);
    conn.setChunkedStreamingMode(16 * 1024);
    conn.setRequestProperty("Transfer-Encoding", "chunked");

    // The "boundry" can be any string. In this example it’s **********.
    // It’s used in the body of the request to seperate each field being submitted.
    //conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);
    conn.setRequestProperty("Authorization", "Bearer " + access_token);
    conn.connect();


    DataOutputStream dataOS = new DataOutputStream(conn.getOutputStream());
    dataOS.write(("json=" + jsonObject.toString()).getBytes(Constants.CHARSET_UTF_8));
    dataOS.write(("image=").getBytes(Constants.CHARSET_UTF_8));

    /* Standard order patten for sending multipart data*/
    dataOS.write(buildStartPayload().getBytes(Constants.CHARSET_UTF_8));
    dataOS.write(getImageBytes(largeImageBitmap));
    dataOS.write(buildEndPayload().getBytes(Constants.CHARSET_UTF_8));

    Log.d(TAG, "Posting String data to server : " + dataOS.toString());

    dataOS.flush();
    dataOS.close();

    // Ensure we got the HTTP 200 response code
    int responseCode = conn.getResponseCode();
    String responseMessage = conn.getResponseMessage();
    Log.d(TAG, "Response code for upload image query : " + responseCode + " Message : " + responseMessage);

    if (responseCode != 200) {
        dialog.cancel();
        Log.e(TAG, String.format("Received the response code %d from the URL %s", responseCode, url));
        // DisplayMessage.error("Couldn't upload image. Please try again later.", activity);
    }

    // Read the response
    InputStream is = conn.getInputStream();
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    byte[] bytes = new byte[1024];
    int bytesRead;
    while ((bytesRead = is.read(bytes)) != -1) {
        baos.write(bytes, 0, bytesRead);
    }
    byte[] bytesReceived = baos.toByteArray();
    baos.close();
    is.close();

    String response = new String(bytesReceived);
    Log.d(TAG, "Response:" + response);

    conn.disconnect();
    conn = null;
    Log.d(TAG, "Cleard the connection handle.");

    return responseString;
}

private static String buildStartPayload() {
    String contentDisposition = "Content-Disposition: form-data; name=\"testing\"; filename=\"testing.png\"";
    String contentType = "Content-Type: image/png";

    // This is the standard format for a multipart request
    StringBuilder requestBody = new StringBuilder();
    requestBody.append(LINE_START);
    requestBody.append(BOUNDARY);
    requestBody.append(LINE_END);
    requestBody.append(contentDisposition);
    requestBody.append(LINE_END);
    requestBody.append(contentType);
    requestBody.append(LINE_END);
    requestBody.append(LINE_END);
    return requestBody.toString();
}

private static String buildEndPayload() {
    // This is the standard format for a multipart request
    StringBuilder requestBody = new StringBuilder();
    requestBody.append(LINE_END + LINE_START + BOUNDARY + LINE_START + LINE_END);
    return requestBody.toString();
}

I used HttpURLConnectionto post an image with multiple parts / forms and tried to do this with DataOutputform, but I get a "Bad Request" error. Is there a library that can help? I use the main salvo, but it does not have good image support. I have not tried Retrofit, but I do not want to go there right now. I hope this can be done using HttpURLConnection.