Originally posted by žane
The reason for this is because they ARE bitmaps, IE partially downloads the file to examine the header and determine the file type, it's why sometimes you have that short pause before the download window is shown.
It appears random because it's not often people rename their bitmaps with a .jpg extension.
To prove this, upload a bitmap with a .jpg extension or vice versa and attempt to download it.
I am just gonna have to disagree with that completely. I have downloaded pics from websites here at work and have IE save them as BMP, and then go home and re-download the same exact file and it be in the correct gif or jpg format. And not only is IE clueless as to what the file format is in such cases, it can't even discern the name of the file, meaning that everytime this happens, you are left with Untitled.bmp When this happens, I can open the source and see that the pic indeed has a name and proper extension. I could understand if there was some unique thing in the jpeg header that IE may not be familiar with, but it is able to display it. And why would IE just 'lose' the filename, while Netscape and Opera neither have a problem with either?