Posting this for my own reference later. When your email client receives a winmail.dat attachment it means that the sender used MS Outlook and your email client can’t handle it.
Here you can download a program to decode that file. Extract the file to your desktop. Drag the winmail.dat file on to the VMDecode program. It will extract the files to the location of the winmail.dat file.
The author provides a fixed expiration time limited demo on the site (you can go back as many times as you’d like for a new one). He sells an unlocked version for $10 for his kids college fund.