"Content-Disposition is an optional header; In its absence,
presentation should default to 'inline'."
What is the justification for this? To me 'attachment'
seems more natural and will fit with many existing viewers.
I'd rather amend this to say:
The Content-Disposition header is optional. In the absence
of a Content-Disposition header, the manner in which the
body part should be presented is undefined.
This sounds fine to me.
Actually, I think it's out-of-scope for the Content-Disposition
document to define what happens when the header is not present.