fix(files): Fix folder preview for favorites widget

[provokateurin]

Summary

Folders don't have a preview and result in 404.
Alternatively we could change the endpoint to also generate "previews" for folders when mimeFallback=true.
That would make it more robust overall which would be nice.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

/backport to stable31

[susnux]

Alternatively we could change the endpoint to also generate "previews" for folders when mimeFallback=true.

👍 I think if the fallback was requested then we should also add that fallback (folder icon).

[provokateurin]

Ok, then I will add this as an enhancement on master, but not backport it as a fix for 31.