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.

The --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 .frm, .myi, .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.