Configure Computers Registry -- Change .DAT extension to .TXT?

I understand how to change the extension of a file with .DAT to .TXT. But, I deal with .DAT files a lot and I am tiresome of continually changing .DAT to .TXT on numerous files. That said, how do you change your registry to make sure that it reads all downloaded .DAT to make it read that it is a .TXT file.