However, this raises a question: does *anyone* use external-body in
association with I-D announcements?
Several MUAs support message/external-body, but they don't all work with
the I-D announcements. Specifically, some MUAs only render the entities if
a Content-Transfer-Encoding MIME header field is defined,
How bizarre. You mean those MUAs can default to 7bit for other bodyparts
but not for message/external-body? And that multiple implementors have
made this same error?
Even though fixing the MUAs would be the best fix in the long-term, adding
the CTE MIME header field to these entities would at least allow more MUAs
to render the entities appropriately. Since this is a mandatory header
field for some entities, there is some argument that the missing CTE is
the problem anyway.
The RFC is quite clear that in the absence of a content-transfer-encoding
field it is interpreted as 7bit. And while for some types it is a
practical necessity (try encoding a JPEG image using only octets between
0x01-0x7f with 0x0d 0x0a pairs every 80 or so octets) as far as
the MIME standards are concerned it's not mandatory for any kind of body
part. Certainly it's not needed for message/external-body.
But I do think it would be an interesting experiment to add the line
content-transfer-encoding: 7bit
to the external body parts of those notices, just to see how many more
MUAs worked with them.
Keith