Just for discussion, because Skwire is the resident minor deity, from the type of change you wanted to post, one "low tech" way to do it is to "hide extensions". Then when you go to change the file name, where it's all selected, hit Right Arrow and it drills over to the right side of the file name, at which point you type your new characters and hit enter.
One of my first "announce your software" I posted a while ago was a "widget-thingie" for XP that 1-click toggled extensions on and off because I did that all day long.