[stable28] fix: Limit open with Collabora action to pdf files

[backportbot[bot]]

Backport of #4231

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.