[stable29] feat: add confirmation dialog for file extension changes

[backportbot[bot]]

Backport of #49308

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

Todo

• [ ] Review and resolve any conflicts

• [ ] Remove all the empty commits

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

[nfebe]

Not backporting