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, 0 deletions

diff --git a/doc/lispref/errors.texi b/doc/lispref/errors.texi
index 45e7acbaa6b..cf9b67c80d8 100644
--- a/doc/lispref/errors.texi
+++ b/doc/lispref/errors.texi
@@ -215,6 +215,9 @@ The message is @samp{Wrong number of arguments}.  @xref{Argument List}.
 
 @item wrong-type-argument
 The message is @samp{Wrong type argument}.  @xref{Type Predicates}.
+
+@item unknown-image-type
+The message is @samp{Cannot determine image type}.  @xref{Images}.
 @end table
 
 @ignore    The following seem to be unused now.