.dat file extension is used by many programs and it doesn’t have s predefined data structure unlike say a .pdf. There could be anything in there from graphics, text or just binary data.

The best thing to do is contact your boss and ask him what it is and what program he used to create it.

An alternate strategy is to open the file in a text editor. You will be presented with a page (well probably hundreds of pages) of nonsense. If you look at the first line of the file you may be lucky enough to see what program created it.

If you’re talking about winmail.dat—that’s a TNEF file from MS Outlook.

Go download this program: http://www.joshjacob.com/macdev/tnef/download.html

And it’ll straighted you right out.

It was a winmail.dat file. I got the TNEF program and it worked! Thanks!!!