[stable25] Check download attribute when fetching previews

[backportbot[bot]]

Backport of #2434

Warning, This backport's changes differ from the original and might be incomplete ⚠️

Todo

• [x] Review and resolve any conflicts

• [x] Amend HEAD commit to remove the line stating to skip CI

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[artonge]

Error from PHPUnit tests:

This version of Nextcloud requires at least PHP 8.1
You are currently running 8.0.30. Please update your PHP version.

I guess this can be ignored, and the PR forced merged.

[artonge]

Pushed workflow update to use stable25 instead of master