Try this:

Open the TB profile:

- TB menu > Help > Troubleshooting information

- under the Application Basics section next to Profile Folder, click 'Open folder'

- close TB > rename the handlers.json file in your profile folder

- restart TB > go to TB menu > Help > Troubleshooting Information > click the 'Clear startup cache' button

After restarting, when you try to open one of those attachments, you should be presented with a screen where you can choose which app to use.

If that didn't work, try this:

Open the config editor:

- TB menu > Settings > General > scroll all the way down > click the 'Config editor' button

Search for and set these to true:

- network.protocol-handler.warn-external.http

- network.protocol-handler.warn-external.https

Then try to open one of those attachments. If it works, set the two options back to false.

The first suggestion worked. Thanks for your help.