lol, that was weird
hmm, I suppose you're using gzipped output on the page?
(if you rename the unpacked file to be a zip, you can unzip that one and get all the files ... anyhow, I need to test on my forum to see if it's the same ... I'll report back later today)
Edit:
Crap, happens on my forum aswell if I use gzipped output
As a temporary fix, disable the gzipped output in Admin-Options
I'm going to look further into this and hopefully be able to come up with a fix later (oddly enough it is ok with firefox, so I'll see what I need to do ... hopefully just make a small change inside attachment.php