Your default mail client Mail for Gmail doesn't support attachments.

Hello totafromind,

Welcome to Apple Support Communities!

It sounds like you may be having some trouble with attachments in the Mail for Gmail app. We'd like to help.

We see that you're coming to us from Change the default web browser or email app on your Mac.

Quit apps on Mac - Apple Support -- Quit the Mail for Gmail app and open it again. See if you're able to use attachments now.

Log out, sleep, wake, restart, or shut down your Mac - Apple Support -- If you still can't use attachments, restart your Mac and test.

How to delete apps on your Mac - Apple Support -- If you're still getting that warning, delete the Mail for Gmail app and reinstall it.

How to contact an app developer - Apple Support -- If you're still unable to use attachments, contact the developer for more help.

Take care.