code: def get_task_descriptor_extension(self, courseid, taskid):
          
          if not id_checker(courseid):
              raise InvalidNameException("Course with invalid name: " + courseid)
          if not id_checker(taskid):
              raise InvalidNameException("Task with invalid name: " + taskid)
          descriptor_path = self._get_task_descriptor_info(courseid, taskid)[0]
          return splitext(descriptor_path)[1]
docs: :param courseid: the course id of the course
              :param taskid: the task id of the task
              :raise InvalidNameException, TaskNotFoundException
              :return: the current extension of the task descriptor