[stable26] 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.