Add codes that convert from emacs unsupported formats to inline images

Sometimes, I receive mails attached Windows Bitmap (bmp) files.
Most emacsen don't support bmp image format.
So, the emacsen can't show the bmp files to inline images.

(Maybe, Meadow supports bmp format and can show it to inline images. But, most emacsen can't.)

This patch adds codes that convert from emacs unsupported formats to inline images.
Bmp etc. to pnm / pam by netpbm image converting tools.