Joe -
It just so happens that I did some initial testing with those very files when I first developed this app. And they worked then, but I just redownloaded and they don't work now.
It also just so happens that I still have the original files I tested with, and they still work (attachments and all). On the "My Test Winmail.dat File" from the Sourceforge page, the new download file is 8 bytes bigger than it was 6 months ago. The "new" file is identical, just with 8 extra bytes at the end. Without the 8 extra bytes, everything works fine.
I'm going to send both the old and new files to the JTNEF developer to see if he wants to take a look. In the meantime, you might want to see if you could get an Outlook/Exchange user to generate some files for you and see if those work better.