Variable: image-type-header-regexps

Alist of (REGEXP . IMAGE-TYPE) pairs used to auto-detect image types.
When the first bytes of an image file match REGEXP, it is assumed to
be of image type IMAGE-TYPE if IMAGE-TYPE is a symbol. If not a symbol,
IMAGE-TYPE must be a pair (PREDICATE . TYPE). PREDICATE is called
with one argument, a string containing the image data. If PREDICATE returns
a non-nil value, TYPE is the image's type.