One of my share extensions has disappeared -- how can I get it back?

I use the MailMate email program. I have always been able to select "MailMate" from the share options on the Finder, and use it to send an email message with the chosen file as an attachment.

But now that item is gone from my menu. I checked the relevant Extensions preferences page -- System Preferences > Share Menu and the MailMate item is not deselected: it is simply no longer there.

What should I do to restore this?

Is there any chance that it was removed when I updated to Monterey with its new security limitations?