--- mm-decode.el~	2018-02-28 02:01:37.897607000 +0000
+++ mm-decode.el	2018-03-13 02:23:04.321753900 +0000
@@ -1810,6 +1810,11 @@
       (when (and (or coding
 		     (setq coding (mm-charset-to-coding-system charset nil t)))
 		 (not (eq coding 'ascii)))
+	;; Remove extra bytes in utf-8 encoded data.
+	(when (eq coding 'utf-8)
+	  (goto-char (point-min))
+	  (while (re-search-forward "[\x00-\x7f]+\\([\x80-\xbf]\\)" nil t)
+	    (replace-match "\\1")))
 	(insert (prog1
 		    (decode-coding-string (buffer-string) coding)
 		  (erase-buffer)