The Agg backend has a feature where if "bmp" was specified as a file extension, it saves as a raw RGBA image. IMHO, this is perhaps too easily confused with the "Microsoft Windows Bitmap" format. There is a (someone else's) bug filed against this:
As part of my recent refactoring of code to map image filetypes to backends, I removed this meaning of .bmp. If anyone has any reasons why .bmp should stay as a "raw" format, let me know, and I'll put it back in. Either way, I'll close the bug as either "fixed" or "rejected".