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.

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[max-nextcloud]

I had another look at this today. When originally introduced the 'prevent download' option was only available for office mime types:
https://github.com/nextcloud/server/pull/32482/files#diff-a767a3e36d22b92db810b48aae05bc3cad43c31de6afb17e926ad1dee6f4da31R425-R439

I just tried and now i can also apply it to markdown documents. Might make sense to investigate when and why this change happened.

[max-nextcloud]

When originally introduced the 'prevent download' option was only available for office mime types:

I had another look at this today with @juliusknorr - turns out the option always blocked all downloads and viewing - it was just using a wrong title on the setting.

We came up with a longer term plan, the first step of which would be the creation of a boolean setting that changes the behavior of internal files regarding disable downloads to the one of public shares with their hide download setting.

Make this setting default to hide download on new instances. If possible only set it to prevent download on existing instances if there are internal shares with download disabled.

I'm not sure if this would be a meaningful way to continue this PR. Keeping it open for now.