I had a user report to me today that jpg image uploads were not functioning. I had him send me the images so that I could try uploading them to my site. They worked just fine. So what's the problem? Some genius over at Microsoft decided that instead of using image/jpeg as the mime type for JPG's they would use image/pjpeg.
If you're checking mime types for your image uploads, be sure to include image/pjpeg in the list or you'll find yourself dealing with bug reports.