I seem to have discovered a bug when asked "What should Thunderbird do with this file?" I have checked the "Do this automatically..." However, it does not get added to the Preferences, Files & Attachments list. This impacts all file extension not already in the list. So TB will open a .doc in Word, but not a docx. Same with any other file not in the list. TB is not adding the extension and association to the list. Everytime a docx comes in (or anything I've already told TB what to do, the "What should TB do...?" opens.