.DAT files is a file has arbitrary data and is not associated with any one particular program or application. When you see a file with a .XLS extension, you know it’s referring to an Excel file, and so on. But with .DAT files, you have to figure out how to open it yourself and it may not be the same program each time.
The best way to open a .DAT file is to use the program that created it. You may want to ask the sender to send it in a different format, like .pdf.
Winmail.dat files come from those Microsoft Outlook or Exchange programs and are some sort of archive method. Somewhere in outlook they get automatically created when forwarding email messages and Mac users have long wondered what they are and how to open them. Of course Windows users have no idea what they are either and think it’s all the receivers issue when we say we never got an attachment.
You could also try this software - http://www.joshjacob.com/mac-development/tnef.php