Notes: None of us need ever to send OOXML. TXT is not a good idea because then the question of character set arises.
for most uses (where formating is not important) txt inside the body of the email is the best option. makes it easy to quote etc... character set is also not much of a problem today since the character set is described in the MIME header and most programms can deal with all of the character sets. although UTF-8 encoding should be preferred today..
so i would list TXT as "ideal" (for most pruposes).
mond.