mml-generate-default-type

mml-generate-default-type is a variable defined in `mml.el'.
Its value is

"text/plain"

Documentation:
Content type by which the Content-Type header can be omitted.
The Content-Type header will not be put in the MIME part if the type
equals the value and there's no parameter (e.g. charset, format, etc.)
and `mml-insert-mime-headers-always' is nil. The value will be bound
to "message/rfc822" when encoding an article to be forwarded as a MIME
part. This is for the internal use, you should never modify the value.