Originally posted by Twistin' Ghost
IE has a bug that they will never fix where it randomly thinks pics are bmp format, so you just have to live with that.
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.