Well, for one thing, it's implicit in the first conformance requirement for
text when the charset is US-ASCII:
-- Recognize and display "text" mail with the charset "US-ASCII."
This doesn't say anything about the subtype being "plain".
But then there's an explicit conformance requirement covering this case:
-- For unrecognized subtypes in a known charset, show or offer to show
the user the "raw" version of the data after conversion of the content
from
canonical form to local form.
How does converting the text/paragraph entity to an attachment violate
either of these constraints?
It doesn't. But that's not the issue at hand -- the issue was whether or not
agents are allowed to not have any display option for such material at all.
They aren't. If they also want to have an option to treat it as an attachment
that's fine, and if they want to make treat as attachment the default that's
fine. But it cannot be the only option.
Ned