Not all attachments are attachments ;-)

I don't think LISTSERV 1.8d considers .TXT as attachments.  Additionally
non-MIME encodings aren't considered either, though some MUAs may
auto-decode them.

1.8e I recall is a little more strict, including its interpretation of

LANGUAGE= NOHTML

Yup as you would guess, all of this is documented and some has been discussed.

Pete