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Re: disposition vs. media type

2006-09-19 00:45:52

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

Where they are specified.  Your objection was to my remark
about a part with no Content header field at all.  In that
case it's either the default message/rfc822 for a part of
multipart/digest, or default text/plain US-ASCII 7bit for
all other multiparts (RFC 2046 5.1.1)

For a simple message/rfc822 without Content header fields
that's ordinary mail, no tricks, no traps, no attachment -
unless you and your MUA wish to look for say UUE, that's
your private business.

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

An unrecognized CTE isn't the same as no explicit CTE.  The
default is 7bit (RFC 2045 6.1).  For unrecognized subtypes
of text the default type is text/plain (RFC 2046 4.1.4), and
any charset other than US-ASCII must be specified (4.1.2).

 From that I guess that UAs could be free to panic if they
find  an 8bit octet in wannabe-default text/plain US-ASCII,
and enter "unrecognized CTE mode".  But that's not what you
found in RFC 2049, where "unrecognized" IMO means "the UA
doesn't know the explicitly specified CTE", e.g. CTE x-yenc.

Clearly there must be an explicit CTE as soon as there are
8bit bytes in mail (RFC 2046 4.1.2 on page 8)

The same section, paragraph "(6)" states that if an
unrecognized charset is specified, text media types should
be treated as application/octet-stream

Yes, I'll test it with this message, let's see what our
UAs do.

RFC 2046 states that behavior only if the subtype is
also unrecognized

Again 4.1.4, I see it.  With that text/foo; charset=bar
is a convoluted way to get application/octet-stream.

If that's an intentional difference from RFC 2049 I miss
the point.  An attempt: text/html; charset=bar  For a
recognized text subtype there could be a more elaborated
fallback, e.g. Latin-1 in that case.  Because RFC 2049
discusses "minimal conformance" such cases are out of
scope.

Perhaps you consider MIME-conforming behavior to be broken?

I just don't confuse unrecognized and unspecified... <beg>

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.

If specified.  Otherwise there are sound defaults with the
unsurprising effect to display plain text ASCII as plain
text ASCII.

application/octet-stream with a disposition of "inline"
doesn't make much sense.

That's up to the UA, it could start a hex. viewer <shrug />

In reality we want it to ask if it should save this as a
file, if we care to name an application to do something
with it, or if we want to just ignore / skip / trash it.

And ideally (in my parallel universe) it's never accepted
by the MSA or MX, let the sender figure out how to get it
right.

US-ASCII text/plain with a specified disposition of
"attachment" should never be displayed inline by a
2183-conforming UA.

Certainly it shouldn't without asking, if it supports
RFC 2183 (not covered by RFC 2049 for obvious reasons).

That's again a case of a _specified_ disposition, and
your objection was to my remark about no Content header
field at all.

Apparently that's the definition of an "attachment" in
RFC 2183:  no automatic display without asking.  Display
after asking is okay.  Makes sense, e.g. for a text/html
attachment you could decide to display it as plain text.
For some attached text/plain source code you could wish
to look at it before you save it as file.  For ordinary
mail worms checking if the raw B64 starts with TV or UE
(= boring, delete) is also okay.

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

That's IMO not unusually safe but unusually paranoid.

If an UA runs on top of a terminal emulation where weird
escape sequences can do bad things, then the "security
considerations" for the spammers would be "if you try
this add an explicit Content-Disposition: inline".  Zero
advantage for the users of this paranoid UA, but more
gibberish annoying users of pre-MIME UAs and/or modems.

it apparently is beyond the sensibility of some UA
programmers to avoid decoding and displaying mislabeled
HTML, including scripting content

True, but my vision of displaying plain text is "as is".
If possible without crashing at NUL bytes (my UA does
this).  Wrt NO-WS-CTL we discussed this already for some
years "elsewhere" (BTW, get your LC comments in shape :-)

Frank


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