In the Preferences/General/Files & Attachments section, delete the action for one of the content types, e.g. jpg, restart TB, then open an attachment and follow the prompt to set the opening application. If selecting an app from the presented list doesn't work, click Other.. and select the app directly. The settings for all file types is in the handlers.json file in the profile folder (Help/Troubleshooting, Profile Folder), so if you delete or rename that file, while TB is closed, a new file is created on restart, and new entries are made when you define the opening app, as above.