dat files can also represent many number of things across multiple platforms. I can send you a dat file containing data used in MATLAB and your "simple Windows machine" would freak out and now be able to do anything while my MacBook Pro, with MATLAB, would be able to work with the file just fine.
iOS itself work with various attachements without issues. You can view jpeg images, PDF files, and a bunch of other content natively without the aid of an app. There are many other attachements that iOS, just like Windows, requires an app to view. Try downloading an mpeg-4 AAC audio file, that was attached to an e-mail, under Windows XP. It isn't going to work without the aid of a program such as Winamp, iTunes, foobar2000, or some type of WMP plug-in. So opening e-mail attachements under a "simple Windows machine" isn't the poster child it was made out to be as there are plenty of examples where it too requires the use of a program (or app in iOS language) to open the file.
As previously pointed out, the use of dat files (particularly when it comes to e-mail) is rather outdated and Microsoft recommends not using it anymore. I use dat files on a daily basis simply because the software I use (which is hooked up to an instrument for measuring a water quality parameter) can output data in either a dat file (which works with MATLAB) or some strange file format that I have never seen and MATLAB freaks out over (I think it is a proprietary format used by that software only).