Return a list of image types supported by ImageMagick.
Each entry in this list is a symbol named after an ImageMagick format
tag. See the ImageMagick manual for a list of ImageMagick formats and
their descriptions (http://www.imagemagick.org/script/formats.php).
You can also try the shell command: `identify -list format'.
Note that ImageMagick recognizes many file-types that Emacs does not
recognize as images, such as C. See `imagemagick-types-enable'
and `imagemagick-types-inhibit'. (fn)