[imp] Differences between multipart/mixed and multipart/alternative

[Otto Stolz]

Hello Len,

you have written:
> emails received
> with a
> Content-Type of "multipart/alternative" for some reason will not display an
> attachment or indicate that an attachment is there.

The reason is that it should be like this: “multipart/alternative”
means that there are several alternative presentation forms (like
different languages or different data formats) for essentially
the same information; hence the MUA should present only one of
these alternatives to the user. Cf. RFC 2046, section 5.1.4
<http://rfc-ref.org/RFC-TEXTS/2046/chapter5.html#sub1sub4>.
Another short example can be found in:
<http://www.systems.uni-konstanz.de/Otto/Vortrag/E-Mail/Kopfzeilen.html#MM-alternative>.

Hence, rather the sending software should be fixed.
Still, you can tell Imp to display the alternative parts to the user,
as you have described already; this would help to cope with broken
software on the sending side, but will probably confuse the readers
of the popular multipart/alternativ(text/plain+text/html) type
messages.

Best wishes,
   Otto Stolz