use its file extension.as image type.
Optional DATA-P non-nil means FILE-OR-DATA is a string containing image data.
Optional PROPS are additional image attributes to assign to the image,
-like, e.g. `:heuristic-mask t'.
+like, e.g. `:mask MASK'.
Value is the image created, or nil if images of type TYPE are not supported."
(when (and (not data-p) (not (stringp file-or-data)))
(error "Invalid image file name `%s'" file-or-data))