private List<CustomFile> taskList;
private AsyncTask fileTask;

@Override
public void onCreate() {
    super.onCreate();
    taskList = new ArrayList<>();
    fileTask = new fileTaskAsync();
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {

    String filePath = intent.getStringExtra("filePath");
    String fileType = intent.getStringExtra("fileType");
    String taskType = intent.getStringExtra("taskType");
    String fileName = intent.getStringExtra("fileName");

    CustomFile customFile = new CustomFile();
    customFile.filePath = filePath;
    customFile.fileType = fileType;
    customFile.taskType = taskType;
    customFile.fileName = fileName;
    taskList.add(customFile);

    Notification notification = getNotification();

    startForeground(787, notification);

    if (fileTask.getStatus() != AsyncTask.Status.RUNNING) {

        CustomFile current = taskList.get(0);
        taskList.remove(current);

        fileTask = new fileTaskAsync().execute(current);
    }

    stopSelf();
    return START_NOT_STICKY;
}

@Nullable
@Override
public IBinder onBind(Intent intent) {
    return null;
}

private class fileTaskAsync extends AsyncTask<CustomFile, Void, String> {

    @Override
    protected String doInBackground(CustomFile... customFiles) {

        CustomFile customFile = customFiles[0];

        FileUtils.doFileTask(customFile.filePath, customFile.fileType,
                customFile.taskType);

        return customFile.fileName;
    }

    @Override
    protected void onPostExecute(String name) {
        sendResult(name);

        if (!taskList.isEmpty()) {
            CustomFile newCurrent = taskList.get(0);
            taskList.remove(newCurrent);
            fileTask = new fileTaskAsync().execute(newCurrent);
        }
    }
}

private void sendResult(String name) {
    Intent intent = new Intent("taskStatus");
    intent.putExtra("taskName", name);
    LocalBroadcastManager.getInstance(this).sendBroadcast(intent);
}