Retrieve from the media colection.
        HTTP GET on /media
        :param external_id: A unique identifier in an external system, set in metadata on POST.
        :return: list(VoicebaseMedia)

def list(self, external_id=None):
        """
        Retrieve from the media colection.
        HTTP GET on /media
        :param external_id: A unique identifier in an external system, set in metadata on POST.
        :return: list(VoicebaseMedia)
        """
        response = self._get('base')
        return [VoicebaseMedia(o, self.api) for o in response['media']]