bug#56906: 28.1; attachment names containing Chinese characters are handled wrong by default

[Lin Jian via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> I'm unable to reproduce this.

I am willing to help you to reproduce this issue. Feel free to ask me
for information you need.

> It looks to me like something has double-encoded the header, so probably
> an SMTP server somewhere that's mishandling the mail?

I don't think it has something to do with SMTP server because I can see
the double-encoded header locally before sending an email if I add some
"print" command around message-encode-message-body and
mail-encode-encoded-word-buffer in message-send-mail like:

(message "debug1: before message-encode-message-body\n%s" (buffer-string))
(message-encode-message-body)
(message "debug1: after message-encode-message-body\n% (buffer-string))

(message "debug2: before mail-encode-encoded-word-buffer\n%s" (buffer-string))
(mail-encode-encoded-word-buffer)
(message "debug2: after mail-encode-encoded-word-buffer\n%s" (buffer-string)))

In the *Message* buffer, after "debug1: after message-encode-message-body", I can see:

Content-Disposition: attachment;
 filename="=?utf-8?B?5rWL6K+VMTIzYWJjLnRhcg==?="

and after "debug2: after mail-encode-encoded-word-buffer", I can see

Content-Disposition: attachment;
 =?us-ascii?Q?filename=3D=22=3D=3Futf-8=3FB=3F5rWL6K+VMTIzYWJjLnRhcg=3D=3D?=
 =?us-ascii?Q?=3F=3D=22?=