Recently I installed an encryption key and Enigmail extension on Thunderbird in order to be able to sign my mails electronically and to check my correspondant's signatures. Everything went ok, apart from the fact that all the mails in my mailbox lost the mime attachments, I only could read the text part...very annoying!
Go to Thunderbird Preferences/Advanced/Config Editor and search for:
mailnews.display.html_as set it to 0
mailnews.display.prefer_plaintext set it to false
You don't need to restart Thunderbird, your messages will come alive again...