How do I open a file with an extension of .xpx?

Here are the apps that use that extension:
http://filext.com/detaillist.php?extdetail=xpx&Search=Search

Of the three listed, my guess is that it's a Dungeons and Dragons file.

I found the following info about xpx extensions:

http://filext.com/detaillist.php?extdetail=XPX

However, the question becomes where did you get or encounter this file? Usually that gives a clue. If it was sent to you by someone, then ask them. Many files were NOT meant to be opened.