Hey,
if you’re a bit old-fashioned like me you’re still using a desktop mail client like Thunderbird and not web interface, like Gmail.
But sometimes there are glitches. And while some of them are client related, others are well - MS fault.
Like this one. For some time now I’ve been getting some emails from the same source with an attached winmail.dat file. There was no way to open it and frankly, knowing the source, I didn’t care too much.
But a few days ago I logged in to Gmail’s web interface and saw the actual file behind it - it was a pdf. Now, when I can’t get a pdf by email, I start to get a bit nervous. So I googled it and came to a conclusion that it’s, of course, MS fault. See, their outlook sends out some emails in tnef format, which can’t be read by some clients.

There are few solutions. For every OS actually. Some are scripts, some are add-ons, some are apps. At first I installed the tnef program from the repositories in Ubuntu, but it did nothing, so instead of starting digging I just installed the LookOut add-on for Thunderbird and voila! I see the original pdf as the attachment.
Now, in case it doesn’t work for you, maybe you should try this:
http://blog.andrewbeacock.com/2007/07/opening-winmaildat-tnef-files-in.html

Hope it helps!

Ozzik.