Occasionally in Mac OS, I’ll get an e-mail with a .dat file extension. .dat is pretty generic, but typically this is from an older Microsoft e-mail program bundling the message and one or more attachments into the .dat. The Microsoft program knows what’s there, but the Mac doesn’t.
Save the .dat file out of the e-mail, then drag and drop it on top of TNEF’s Enough. This great little utility makes extracting attachments from .dat files quick and easy.