disposition vs. media type (Was Re: MIME part header)

On Mon September 18 2006 03:29, Frank Ellermann wrote:
> 
> Bruce Lilly wrote:

> > A MIME-part has inline disposition if and only if it contains
> > a Content-Disposition MIME-part header field specifying
> > inline disposition.
> 
> I'd consider a MUA not displaying US-ASCII text/plain as plain
> text as broken, YMMV.

Media type (text/plain, etc.) and disposition (inline, attachment, etc.)
are distinct characteristics of a MIME-part.

Incidentally, RFC 2049 specifies that MIME-conforming UAs must treat
text/plain (and all other media types) with an unrecognized transfer
encoding as application/octet-stream [sect. 2, paragraph labeled "(3)"].
The same section, paragraph "(6)" states that if an unrecognized charset
is specified, text media types should be treated as application/octet-stream
(RFC 2046 states that behavior only if the subtype is also unrecognized).

So an MUA which displays US-ASCII text/plain with an unrecognized
content transfer encoding as application/octet-stream (rather than
text/plain) is MIME-conforming, and one which displays it as text/plain
is not MIME-conforming.  Perhaps you consider MIME-conforming behavior
to be broken?

So while there is interaction between transfer encoding and media type,
and between charset and media type for text subtypes, disposition and
media type are orthogonal.  Mind you, the expected behavior for
application/octet-stream and for disposition "attachment" is similar;
moreover application/octet-stream with a disposition of "inline" doesn't
make much sense.

Getting back you your statement, Frank, US-ASCII text/plain with a
specified disposition of "attachment" should never be displayed inline
by a 2183-conforming UA.  US-ASCII text/plain with no specified
disposition can be displayed inline or not by an MUA.  One which does
not display it inline would be unusual, but also unusually safe, as it is
not above the ethics of spammers, virus writers and other miscreants to
mislabel other content as text/plain (and it apparently is beyond the
sensibility of some UA programmers to avoid decoding and displaying
mislabeled HTML, including scripting content, putatively on the grounds
that users want to have their computers infected with viruses, worms, etc.),
or to imbed ECMA 48 escape sequences, etc. in text.