I am puzzled a lot when I got emails with winmail.dat attachment, which is unknown file type attachment in my Thunderbird. Here is the solution for this annoying winmail.dat:
Go to https://addons.mozilla.org/en-US/thunderbird/addon/4433, follow it instructions to download the “LookOut” add-on for Thunderbird, and attached files inside winmail.dat will be listed and be recognized and be opened with existed programs.