REST POST request with image binary

Question

I have made a RESTful web service which I am currently hosting on Google App Engine. The web service works well when I test it using Postman, as it provides the appropriate JSON response. When using Postman, I do not provide any authorization (image 1) and I do include an image in the body as 'binary' (image 2).

However, I am not sure how to go about sending an equivalent request in Java for an Android application. How do I go about forming such a request and including the binary version of an image within the request?

Any help would be much appreciated!

Thanks in advance!

Postman screenshot 1

Postman screenshot 2

Below is the code I currently have, but I do not get a response from my web-service, so I assume that I have done something incorrectly in my code:

import android.app.DownloadManager;
import android.graphics.Bitmap;
import android.os.AsyncTask;

import java.io.File;
import java.io.IOException;
import java.net.URL;

import okhttp3.FormBody;
import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

/**
 * Created by OliverM on 17/12/2017.
 */

public class Downloader extends AsyncTask<File, Integer, String> {
    protected void onPreExecute(){

    }

    protected String doInBackground(File... files) {
        OkHttpClient client = new OkHttpClient();

        RequestBody body = new MultipartBody.Builder()
                .setType(MultipartBody.FORM)
                .addFormDataPart("image", files[0].getName(), RequestBody.create(MediaType.parse("image/jpeg"), files[0]))
                .build();

        Request request = new Request.Builder()
                .url("https://----------------.appspot.com/classify")
                .post(body)
                .addHeader("Cache-Control", "no-cache")
                .build();

        try {
            Response response = client.newCall(request).execute();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return "success";
    }

    protected void onProgressUpdate(Integer... progress) {
    }

    protected void onPostExecute(String result) {
    }
}