Not sure what purpose it serves, but Firefox 3 on OS X changes the names of jpg files to jfif, which is the umbrella filetype for JPEG encoded files. As a blogger and collector of pornography involving goats and fish, I find this very problematic. You can, however, create an Automator task to fix your JFIF files on the fly. I’ve attached it to a downloads folder and it seems to work fairly well.
1. Open Automator and choose Custom.
2. Type “rename” into the search box.
3. Drag “Rename Finder Items” into the grey area to the right. Choose “Don’t Add” to the prompt that appears.
4. Change the first drop down box to “Replace Text”.
5. In the Find box type in “.jfif” and in the Replace box type in “.jpg”. Make sure the Ignore Case box is checked.
6. Click on the File menu and choose Save as Plug-in… Type in “JFIF to JPEG” and make sure the Finder option is chosen in the drop down box.
Now whenever you need to change a bunch of images to .jpeg, just select them in the Finder, right click, choose “More”, choose “Automator”, and click on “JFIF to JPEG.”
I have it set as a folder action, but you can also add it to your right click menu. Either way works.