NEWB ALERT....
Sorry guys but I am having difficulty changing my hi_123u1.txt file to hi_123u1.diff
When I go to rename the file it does not display as hi_123u1.txt, it just shows as hi_123u1, I have tried to stick the .diff bit on the end anyway but it does not fool my machine in to believing that it is a diff file. It just sees it as hi_123u1.diff.txt

I know there is probably something really obvious that I am doing wrong but I can't spot it. I am running XPSP2