+If CODING-SYSTEM is non-nil, decode bytes into characters with that
+coding-system.
+
+Interactively, you can supply the CODING-SYSTEM argument
+with \\[universal-coding-system-argument].
+
+The CODING-SYSTEM argument is a historical hangover and is deprecated.
+QP encodes raw bytes and should be decoded into raw bytes. Decoding
+them into characters should be done separately."
+ (interactive
+ ;; Let the user determine the coding system with "C-x RET c".
+ (list (region-beginning) (region-end) coding-system-for-read))
+ (unless (mm-coding-system-p coding-system) ; e.g. `ascii' from Gnus
+ (setq coding-system nil))