code: def get_older_backup(self, encrypted=None, compressed=None,
                           content_type=None, database=None, servername=None):
          
          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)
docs: 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