Thanks, Helen ...
@Debbie ...
From what you've shared so far ... think you are using Linux ... some of the following is for Linux.
Moodle versions prior to 2.6.x still used .zip as backups.
At that version there was an option to turn on experimental compression using gzip. Backup files retained their .mbz extension.
Size could be an indication they are backups, but they could also be videos or other file types.
file -b /moodledata/path/to/file
If that command responds with:
gzip compressed data, from Unix
They are, more than likely, backups (.mbz's)
If that command responds with:
Zip archive data, at least v2.0 to extract
It is a zip file and might be a link created by a teacher in their course
for resource files.
To double sheck do a query on the DB for your Moodle on mdl_files table.
select contenthash,filename,filesize,filearea from `mdl_files` where (`filearea` like "backup" and `filename` like "%.mbz%");
Or
select contenthash,filename,filesize,filearea from `mdl_files`;
(latter will, on a large site with lots of file resources, take some time and probably should be used in non-prime time depending)
If you remove the file from /moodledata/filedir/ (and the subdirectory
structure IF it contains a single backup file, **ALSO** remove the record
for it in mdl_files table. It is not recommended to ever mess with files in moodledata/filedir/ as the only humanly recognizable file in that directory (warning.txt) states:
"This directory contains the content of uploaded files and is controlled by Moodle code. Do not manually move, change or rename any of the files and subdirectories here."
There is a plugin that might help:
https://moodle.org/plugins/report_filetrash
shows compat from moodle 2.5 to 2.9
Are you running cron/task jobs often enough?
'spirit of sharing', Ken