: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

def get_task_descriptor_extension(self, courseid, taskid):
        """
            :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
            """
        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]