I recently downloaded a batch of pictures from the internet. The iMac refused to read those files as JPG so I did a little digging (actually Command-I on one of the files) and found that these particular files were saved with Photoshop CS5.1 for Windows. The following graphic shows what the files look like on OS X 10.8
The next question you're probably asking is “how to I fix this?” The solution is rather simple but will require a 3rd party program unless you like renaming multiple files by hand. Great, if you only have a couple of pictures to fix but if you have a whole bunch then I recommend Name Mangler. I'll explain why.
Simply changing the name of the file to say “waterfall.jpg” won't fix the problem or at least, it didn't for me. What we need to do is change the extension as well. So, instead of JPG we need it to be JPEG. Name Mangler will allow you to make that change. Once, you rename the files with an extension of JPEG..boom! You have your files in a completely readable format by OS X 10.8.
How about Adobe Bridge CS6? I've tried converting with Adobe Bridge and it just wouldn't cooperate. Sure, you could actually view the photos and see how they were saved originally but that's it. If anyone knows of a method that works in Adobe Bridge, please let me know (me at smpmike dot com)