You could make a hotkey or toolbar button which runs rename *.jpeg to *.jpg and click that when you see some .jpeg files.
You could also set up a filter that highlights *.jpeg files in red so you can see if any need renaming more easily, and know when to push the button.
If you really want to automate it, so you don't have to push a button or hotkey, you could use a script. I think that would be a very bad idea, though. For example, if you went into a program's data folder to look at something and renamed some of its .jpeg data files .jpg, it then would not be able to find them the next time you ran it and you'd have no idea why parts of it had stopped working.