feat: Add allowed_view_extensions config node

[Koc]

📝 Summary

It isn't possible to view md files if they were shared without allow downloading. Related to nextcloud/server#48903

🔍 Repoducer

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' => [
    'md',
    // ...
  ]

🚧 TODO

• ...

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

[max-nextcloud]

@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.

[jancborchardt]

Generally I think it should just work and not necessitate another setting or parameter.

What exactly "just work" means for the "Prevent download" vs being able to view though, it is up to @AndyScherzinger @juliusknorr @sorbaugh to define as it is a technical and security question related to both Files and Office.

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[Koc]

@jancborchardt any suggestions how to make it "just works". In this case I guess we should hardcode list of extensions somewhere