lisp/gnus/mm-encode.el (mm-default-buffer-type): Obey the mode hierarchy

(cherry picked from commit e67f03bf35b97d2898b64281215be6717b659839)

diff --git a/lisp/gnus/mm-encode.el b/lisp/gnus/mm-encode.el
index 021c56e26ef86cb56bdabe935e8ba8f240e878d4..ef75b6da6e1087fd66c429e2c2d582bbc0ff8de0 100644 (file)
--- a/lisp/gnus/mm-encode.el
+++ b/lisp/gnus/mm-encode.el
@@ -110,7 +110,8 @@ This variable should never be set directly, but bound before a call to
 (defun mm-default-buffer-type (buffer)
   "Return a default content type for BUFFER, a buffer name."
   (if-let* ((buf (get-buffer buffer))
-            ((eq (buffer-local-value 'major-mode buf) 'diff-mode)))
+            ((provided-mode-derived-p (buffer-local-value 'major-mode buf)
+                                      'diff-mode)))
       "text/x-patch" "text/plain"))
 
 (defun mm-safer-encoding (encoding &optional type)