Jon,
I don't know why you would get both lower-case filename extensions and upper-case extensions. It looks to me that the drive on your colleague's computer is formatted with a case-sensitive file system. This would allow two files with the same name, differing only in case to exist in the same folder. Is there a reason why he needs his disk formatted in this way?
Also, the Finder should see a .xmp and a .XMP file as both being of the same file type. I think Hayo is right and something has added .txt or .rtf to the .xmp files. PM certainly won't do that.
-Kirk