WD Backup and files with extension .quo

WD Backup is not coping files with a .quo extension. Does anyone know a reason for this?

@andrew1001 Look at all of the following.

https://support-en.wd.com/app/answers/detail/a_id/16747/kw/WD%20Backup%20and%20file%20extensions

http://extension.nirsoft.net/quo

https://file.org/extension/quo

Where are you finding this extension .quo on your computer?

The .quo files are generated by a software package I use to produce quotes for my work. The .quo are located in a subdirectory on my C: drive.

I did some further checking and the .quo files are being backed up. Strange thing is I get a notification that they are not backing up but I checked and they are all there in the back up.