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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_older_backup(self, encrypted=None, compressed=None, content_type=None, database=None, servername=None):
 """ Return the older backup's file name. :param encrypted: Filter by encrypted or not :type encrypted: ``bool`` or ``None`` :param compressed: Filter by compressed or not :type compressed: ``bool`` or ``None`` :param content_type: Filter by media or database backup, must be ``'db'`` or ``'media'`` :type content_type: ``str`` or ``None`` :param database: Filter by source database's name :type: ``str`` or ``None`` :param servername: Filter by source server's name :type: ``str`` or ``None`` :returns: Older file :rtype: ``str`` :raises: FileNotFound: If no backup file is found """

        files = self.list_backups(encrypted=encrypted, compressed=compressed,
                                  content_type=content_type, database=database,
                                  servername=servername)
        if not files:
            raise FileNotFound("There's no backup file available.")
        return min(files, key=utils.filename_to_date)