[Regression] Pre-upgrade backups should exclude large data (like media files)

[kemenaran]

Describe the bug

When upgrading Mattermost, the backup performed automatically before the upgrade now includes all files – including all Mattermost media files.

This makes the backup very large (> 10 Go instead of ~200 Mo), and slow.

Steps to reproduce

• Start a mattermost upgrade on a moderately-large instance,
• Look for the "Info : L'archive contiendra environ 10.3GB de données." message, larger than it should be,
• Notice how slow the pre-upgrade backup is.

Expected behavior

The backup should be light, fast, and not include the media files.

Regression

This looks like a regression from #501 ? https://github.com/YunoHost-Apps/mattermost_ynh/pull/501/files#diff-d7c669d8e7971ae1796dc06b149cbfaee4a56384abaeb3bd0a85077e7c3cb141R18 (cc @ericgaspar)