Fixed it. Apparently, Mozilla gives jpg/jpeg files mime type image/jpeg, while IE names them image/pjpeg. I just had to add the case. This is to prevent other file types from (easily) being uploaded. In other words, you can't just rename .exe into .jpg to store it.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram