Nextcloud Photos App; Group-Folder and TIFF

we are unsing nextcloud (26.latest atm) and get pictures from some design people. Those pictures are delivered as TIFF with about 4500x2500 pixels with around 45MB of data.
While storing the pictures in the files app works perfect, there’s some trouble with the photos app, where the marketing-people wants to have a preview of the images in order to select the desired ones for their campaigns.

I found some Ideas about installing the camera-raw app, adding TIF as preview to the config and so on, but none of them seemed to work.
Even so, triggering the preview:generate-all didn’t show them in the photos app.

Technically (if there’s the problem):
User are in LDAP, all files are in Group-Folders and shared to the users according to their LDAP-Groups. But since jpg-files are shown in the photos app (under shared with me), I doubt that using group-folders prevents the photos app from working.

Is there any suggestion? Maybe there’s something in the config (preferred solution)?
Or is there a way to automatically create thumbnails in a subdirectory, which then show up in the photos app (not preferred)? I checked the media-conversion-app, but I guess this will only work on video, not audio files…