Trust me, to convert .bmp into .png is much easier, quicker, better in every imaginable ways then add support for useless .bmp type of images.
Try this experiment: take one .bmp image write down the filesize, then compress it into .zip, write down the filesize of the .zip, then convert that .bmp into .png and compare filesize of .png with filesize of .zip, you'll see that these numbers are very close to each other then to .bmp filesize (depends on image complexity it could be be 2-100 times difference)
The conclusion: be kind to your site's visitors never ever use .bmp images directly on the web pages, it's not only wastes your server's bandwidth but also bandwidth and time of your visitors.
P.S.
You even can convert .bmp into .png or .jpg with MS Paint...