Convert a file id to the file name.

def file_id_to_name(self, file_id):
        """Convert a file id to the file name."""
        sql = 'select name from files where id = ?'
        logging.debug('%s %s', sql, (file_id,))
        self.cursor.execute(sql, (file_id,))
        name = self.cursor.fetchone()
        if name:
            return name[0]
        return ''