Default exotic image formats (like .webp) to image-mode

* doc/lispref/errors.texi (Standard Errors): Mention the new error.

* lisp/files.el (auto-mode-alist): Add a bunch of image suffixes
to the list (bug#37972) based on the output from "gm convert -list
format" (i.e., graphicsmagick).

* lisp/image-mode.el (image-mode): Rewrite to possibly notify the
user about image-use-external-converter.
(image-mode--setup-mode): Factor out into own function and don't
run under `condition-case' as there's nothing here that should
error.

* lisp/image.el (unknown-image-type): New error.
(image-type): Signal that error so that image-mode can offer
sensible feedback to the user.

1 files changed, 3 insertions, 1 deletions

diff --git a/lisp/image.el b/lisp/image.el
index 66fb5fa5fc9..ad2ee6c6071 100644
--- a/lisp/image.el
+++ b/lisp/image.el
@@ -151,6 +151,8 @@ or \"ffmpeg\") is installed."
   :type 'boolean
   :version "27.1")
 
+(define-error 'unknown-image-type "Unknown image type")
+
 ;; Map put into text properties on images.
 (defvar image-map
   (let ((map (make-sparse-keymap)))
@@ -391,7 +393,7 @@ Optional DATA-P non-nil means SOURCE is a string containing image data."
                             (require 'image-converter)
                             (image-convert-p source))))))
     (unless type
-      (error "Cannot determine image type")))
+      (signal 'unknown-image-type "Cannot determine image type")))
   (when (and (not (eq type 'image-convert))
              (not (memq type (and (boundp 'image-types) image-types))))
     (error "Invalid image type `%s'" type))