Wait! No one other than John Myers was suggesting sending binary data under
application/mime! Neither the S/MIME spec nor the application/mime spec
says that you can use binary under application/mime.
The S/MIME spec directly says the opposite.
The S/MIME specification says the opposite for multipart/signed (at least
currently, but note that if application/mime were adopted as-is this
restriction could be removed from multipart/signed), but the application/mime
specification doesn't. Not only does it not forbid the use of binary
internally, it talks specifically about using base64 or quoted-printable at the
outer level, which opens the door to the use of binary inside. And since this
type is clearly composite in nature this is in clear violation of the MIME