Fix 'submit-emacs-patch' MIME type

* lisp/mail/emacsbug.el (submit-emacs-patch): Use the correct
MIME type for patches.  Otherwise, `mm-inline-media-tests' won't
recognize and fontify the patch.  (Bug#79287)

Author: Stebalien

lisp/mail/emacsbug.el

@@ -587,7 +587,7 @@ Message buffer where you can explain more about the patch."
   (message-goto-body)
   (insert "\n\n\n")
   (emacs-build-description)
-  (mml-attach-file file "text/patch" nil "attachment")
+  (mml-attach-file file "text/x-patch" nil "attachment")
   (message-goto-body)
   (message "Write a description of the patch and use %s to send it"
            (substitute-command-keys "\\[message-send-and-exit]"))