Retrieve a list of permissions for a file.

        :param file_id: a spreadsheet ID (aka file ID.)
        :type file_id: str

def list_permissions(self, file_id):
        """Retrieve a list of permissions for a file.

        :param file_id: a spreadsheet ID (aka file ID.)
        :type file_id: str
        """
        url = '{0}/{1}/permissions'.format(DRIVE_FILES_API_V2_URL, file_id)

        r = self.request('get', url)

        return r.json()['items']