Cannot delete file ending with ".bak"

I have a file in the documents folder that was sync’ed to NC from the desktop sync client, which ends in “.bak”, and I cannot now delete the file from Nextcloud.
I have tried deleting it on the desktop and syncing, deleting it from the server file browser, and from the nextcloud web GUI. No joy.
How may I delete this file?
Running NC 16.0.6
Thank you,

Thant’s an esy task, login to the server using ssh, navigate to the relevant directory and delete the file.