Hello Dan,
I rename to .jpg only if the whole filename is changing as well. The reason is that on a few systems it will not work, .JPG = .jpg. You can’t rename a file to the same name. Basically that happens on Windows and that would end on a WordPress that is not really synchronized between its DB and filesystem and the day you migrate to another system it will be really broken and hard to repair.
If you want to force it, you can simply search and replace the occurences of “strtolower” to “” (nothing) in the whole code. There are probably two occurences. Let me know what it does and if it works for you… But to be honest, this is just a visual thing (JPG or jpg is the same from a technical or SEO point of view) so it’s better to keep things working and safe first.