I have Gnus 5.8.8 so forgive me if this is improved in newer versions.

Attaching a file with an unknown character set produces a less than
helpful warning.

To repeat, create an empty file, and populate it by evaluating the
following Lisp snippet with M-:

  (let ((i 0)) (while (< i 256) (insert (char-to-string i)) (setq i (1+ i))))

Save the file. Then start composing a new message, and attach the file
you created with C-c C-a.

I get an MMF spec which looks like this ("disarmed").

  [#part charset="utf-8" type="text/plain" filename="/tmp/8bit.txt"
         disposition=attachment
         description="Test file for unknown character set"]
  [#/part]

I have experimentally added the charset= field, but it didn't help
(also tried "unknown", "utf8", and "iso-8859-1"), and indented and
wrapped lines to make it more readable.

Attempting to send this message gives the dreaded warning:

  8-bit characters are found in the message, please specify charset.

Adding a charset= header to the MMF info does not appear to help, as
indicated above.

MMF preview (M-m P) gives the same warning, in case you want to try
without actually sending any message.

The warning message contains no hints on how to proceed and I cannot
find information about how to specify a character set, or even
anything general about how to handle this error. (I believe I have
stumbled over it in other situations in the past, when a quoted
message I was replying to somehow ended up not having a valid
character set. Maybe I don't remember this correctly, though.)

A workaround in some cases is to visit the file in a buffer with the
correct coding system selected, and attach the buffer with M-m b
instead.

/* era */


Gnus v5.8.8
GNU Emacs 20.7.2 (i386-debian-linux-gnu, X toolkit)
 of Fri Apr  5 2002 on cyberhq modified by Debian
200 main.gmane.org InterNetNews NNRP server INN 2.3.2 ready (posting ok).