(image-type-header-regexps): Rename from image-type-regexps.
Change uses.
(image-type-file-name-regexps): New defconst.
(image-type-from-data): Simplify loop.
(image-type-from-buffer): New defun.
(image-type-from-file-header): Use it instead of image-type-from-data.
Use image-search-load-path instead of only looking in data-directory.
(image-type-from-file-name): New defun.
(image-search-load-path): Make PATH arg optional, default to image-load-path.
Change `pathname' to `filename'.