A quick Google search found this explanation:
Simply put, DAT files contain data in text or binary format. They are typically found as winmail.dat files in email attachments, created by Microsoft Exchange Servers. But many other programs can create .dat files as well. ... First, you could try to change the file extension.Oct 6, 2009
Looks like you have some Windows files. This same search found these sites:
How to Open Winmail.dat Attachment Files on Mac OS X - OS X Daily
Re: How do I open winmail.dat files?
How To Convert a DAT File into A Word Document - MakeUseOf
With a correctly parsed question you can find a lot from a Google search.