I remember seeing this once before, but the cause escapes me.

Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

I just saw this, but for Windows. I don't know if it will work for Linux.

The solution there was to delete the relevant entries in Options: Attachments: Incoming, then restart Thunderbird.

The next time you click on one of the attachments, it should ask which program to open with, and the issue is resolved.

Thank you for these ideas. I think I fixed it myself by re-setting the default applications in the app by going to Thunderbird-->preferences--> attachments. In Linux, to get to your apps you navigate to root-->usr-->bin. In some cases, I re-selected the same app, for example: gimp for gimp. In others, I chose a "close relative," like libreoffice to replace lowriter. It seems that re-setting them made it all work.