By default, all the files in the data directory are included in the backup, so any non-database files in that directory are backed up.
--only-known-file-types option of the
mysqlbackup command limits the backup to only
those files that represent known data files from MySQL or the
storage engines, such as
.mrg, and so on.
(See the full list of
extensions.) By default, the
mysqlbackup command backs up all file
extensions within the data directory, which could include files
produced by many different storage engines. Use this option if
the additional data files from other storage engines should not
be included in the backup, for performance or space reasons.