It’s not just a SU issue.

It’s a general computer thing.

Some systems and languages are case sensible. some are not.
more here :

Basically, SU won’t accept file with capitalized extensions. PNG, JPG or JPEG won’t work.

the solution? simply use lowercase extensions. no need to fire up photoshop, you can just rename stuff.
Why do some softwares still create capitalized extensions, no idea. I know that at some point wordpress had a similar dislike.

You say JPEG / JPG don’t work, and you’re right.
Dave says jpeg / jpg work and he is also right. It’s just a… capital matter.