Since a recent update (4.3.3), my automatic system backups went from around 100 Mb to 40 Gb !! When I have a closer look, they now contain a hook_data_multimedia option.
Until recently my automated backups were around 100 Mb, then turned to 41 Gb
There is still no label in the web-admin : only they key is displayed hook_data_multimedia (41 GB) hook_data_multimedia_desc
As far as I could see it contains the Yunohost Multimedia directory.
The .nobackup works, but it means that the Nextcloud backup not backup those files. Either you backup them twice (and they are huge) or you don’t backup them.
Other options would be:
Manually change Nextcloud Multimedia to a separate directory
Have a Yunohost option to chose if you want to backup the multimedia directory along with the system