Adjust error message in image-mode

* lisp/image-mode.el (image-mode): Even when
`image-user-external-converter' is on, we may get
`unknown-image-type' (bug#39994).  Adjust the error message in
that case.

1 files changed, 17 insertions, 15 deletions

diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index 1bb213c2489..129529542ae 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -614,21 +614,23 @@ Key bindings:
   (if (not (image-get-display-property))
       (progn
         (when (condition-case err
-                (progn
-                  (image-toggle-display-image)
-                  t)
-              (unknown-image-type
-               (image-mode-as-text)
-               (funcall
-                (if (called-interactively-p 'any) 'error 'message)
-                "Unknown image type; consider switching `image-use-external-converter' on")
-               nil)
-              (error
-               (image-mode-as-text)
-               (funcall
-                (if (called-interactively-p 'any) 'error 'message)
-                "Cannot display image: %s" (cdr err))
-               nil))
+                  (progn
+                    (image-toggle-display-image)
+                    t)
+                (unknown-image-type
+                 (image-mode-as-text)
+                 (funcall
+                  (if (called-interactively-p 'any) 'error 'message)
+                  (if image-use-external-converter
+                      "Unknown image type"
+                    "Unknown image type; consider switching `image-use-external-converter' on"))
+                 nil)
+                (error
+                 (image-mode-as-text)
+                 (funcall
+                  (if (called-interactively-p 'any) 'error 'message)
+                  "Cannot display image: %s" (cdr err))
+                 nil))
           ;; If attempt to display the image fails.
           (if (not (image-get-display-property))
               (error "Invalid image"))