Call get content API.

        https://devdocs.line.me/en/#get-content

        Retrieve image, video, and audio data sent by users.

        :param str message_id: Message ID
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: :py:class:`linebot.models.responses.Content`
        :return: Content instance

def get_message_content(self, message_id, timeout=None):
        """Call get content API.

        https://devdocs.line.me/en/#get-content

        Retrieve image, video, and audio data sent by users.

        :param str message_id: Message ID
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: :py:class:`linebot.models.responses.Content`
        :return: Content instance
        """
        response = self._get(
            '/v2/bot/message/{message_id}/content'.format(message_id=message_id),
            stream=True, timeout=timeout
        )

        return Content(response)