Use better attachment defaults for *diff* buffers

* lisp/gnus/mm-encode.el (mm-default-buffer-type): New defun.

* lisp/gnus/mml.el (mml-attach-buffer): Use it.

lisp/gnus/mm-encode.el

@@ -107,6 +107,14 @@ This variable should never be set directly, but bound before a call to
       "application/octet-stream"
     (mailcap-extension-to-mime (match-string 0 file))))
 
+(defun mm-default-buffer-type (buffer)
+  "Return a default content type for BUFFER, a buffer name."
+  (if (and (stringp buffer)
+           (string-match-p
+            (rx (| "*Diff*" "*vc-diff*" "*ediff-diff*" "*ediff-custom-diff*"))
+            buffer))
+      "text/x-patch" "text/plain"))
+
 (defun mm-safer-encoding (encoding &optional type)
   "Return an encoding similar to ENCODING but safer than it."
   (cond

lisp/gnus/mml.el

@@ -1508,7 +1508,8 @@ FILENAME is a suggested file name for the attachment should a
 recipient wish to save a copy separate from the message."
   (interactive
    (let* ((buffer (read-buffer "Attach buffer: "))
-	  (type (mml-minibuffer-read-type buffer "text/plain"))
+	  (type (mml-minibuffer-read-type
+                 buffer (mm-default-buffer-type buffer)))
 	  (description (mml-minibuffer-read-description))
 	  (disposition (mml-minibuffer-read-disposition type nil)))
      (list buffer type description disposition)))