>>>>> In <m2d50c34o4.fsf@sl392.st-edmunds.cam.ac.uk> Leo wrote:

> To see the problem, go to the attachment in this post, hit 'E' and you
> will see garbled characters.

Because the supertype of that part is `application', not `text',
Gnus considers there is no human readable text in it.  It is
decoded only by base64, as the part specifies.

> Content-Type: application/emacs-lisp
> Content-Disposition: attachment; filename=test.el
> Content-Transfer-Encoding: base64
> Content-Description: test.el

OTOH, `2 K i' or `i' on the button shows the decoded Chinese
text, because the `i' command is for the use of ``View As Text,
In This Buffer''.

Only with No Gnus, you can add the coding cookie to such a file
in order to specify the charset MIME parameter.  For examples: