>  I still haven't received any answer regarding this.

Quite possibly because this is documented.

As documented, you can set the content-type character set in the template
by providing a .cs command line, ie,

.CS ISO-8859-7

You simply code the appropriate character set code in place of "ISO-8859-7"
in the example.

>  And additional question - is there a list of messages to which LISTSERV
>adds Content-Type (and to which it doesn't)? Without that info trial&error
>process is a waste of time.

As also documented, if you put 8-bit characters into a template form, LISTSERV
will encode the template form on the fly when it sends it out and that's
when the Content-Type header gets added.  So there is no straightforward
answer to your question other than "all of them, if they contain 8-bit
characters".  If the default character set doesn't work for you then you
need to include a .CS definition in each template form that contains such
characters.

See chapter 9 of either the list owner's or site manager's manual.

Nathan