Not only is the browser the problem, but the web server is. Remember back when PNG's came to town. You didn't have every webserver add image/png automatically. You didn't have every sysadmin find out about it. It took a while, and there's some out there that don't have that. Right now, BZIP2 format isn't being recognized, and is sent application/octet-stream or, even worse, text/plain! (Try pulling a Kernel, and it spews all over your screen)
IE's being mistrusting of the webserver, rightfully so. But then it hides that fact, not letting programmers know about it. Naughty IE!