Weird file format issue (jpg vs jpeg)

I just started a new job, and the computer I'm working on (a Mac Pro running 10.5.8) has the weird habit of changing every file ending in ".jpg" into a file ending in ".jpeg" instead. It wouldn't be that big a deal, except that Adobe Bridge sometimes refuses to show a thumbnail of a ".jpeg" file, so it disrupts my workflow.

This doesn't happen on any of my personal computers (iMac, MacBook Pro, etc.) and frankly, I've never experienced it before. Any idea how I can stop this behavior?