Main body here refers to the (usually initial) part that contains
descriptive material. The current dispositions really don't make
sense in this
context -- such material is inherently inline and not an attachment
This is sort of contemplating your navel here, but a
content-disposition of attachment on the main body does make logical
sense, it's just that most MUA's don't let you compose such a beast.
If the Content-Type of the message as a whole is Image/gif, then it
certainly makes sense to indicate that the disposition of that gif
object might be attachment rather than inline.
So why should this be considered "undefined"? Granted, MUA's that
enforce a model of message plus attachments might need to do some
dancing to represent this to the user, but it's certainly legal MIME.