3.6 Content-Disposition and the Main Message
It is permissible to use Content-Disposition on the main body
of an [RFC 822] message. Althouth the meanings of the two
current dispositions (`inline' and `attachment') are
respectively vacuous and undefined, it is anticipated that
future dispositions might be more amenable for use with main
messages (one might imagine a "print" disposition to
implement a print-by-mail service, for example).
A disposition of attachment with a main body makes sense to me.
Suppose the main body is a binary data file of some sort. It would
make sense not to automatically display it to the user.
Both inline and attachment make sense with the main body if a file name