feat: Add allowed_view_extensions config node

[Koc]

Summary

It isn't possible to view jpg files if they were shared without allow downloading. Related to nextcloud/text#6564

🔍 Reproducer

Share file without download possibility

Try to view file as share recipient

To fix that we can introduce a new allowed_view_extensions config node.

  'allowed_view_extensions' => [
    'jpg',
    'jpeg',
    'png',
    // ...
  ]

TODO

• ...

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)

[max-nextcloud]

I added a comment in the text PR - crossposting here.

@Koc Thanks a lot for addressing this. The current state is really confusing.

I'm not sure your proposed solution is really what we want. Before diving into the code I think we should clarify that with @nextcloud/designers .

We're always hesitant to add config options. However this config would not be exposed in the UI I assume. But the problematic scenario would remain for files which are not part of the allowed_view_extensions config.

I know the permission setting to prevent downloads is messy. I personally think that this should not exist for the filetypes you list as one can never prevent a screenshot or a copy and paste download. But I'll wait to hear what the designers say.

[susnux]

I think this needs some discussion, as this allows to prevent the use case this settings was meant for.
And also I think this implementation is a bit dangerous as it will allow the download using the WebDAV endpoint, so the whole point of "prevent download" is circumvented.