The type field has NOTHING to do with the transfer encoding used. It might
suggest an appropriate transfer encoding to use -- maybe.
Take the simplest possible case -- type text. Text might be encoded using
quoted-printable. But if the text is in a character set that happens to use
mostly 8-bit characters, such an encoding is both wasteful and useless; base64
would be better.
In the case of mostly 8-bit character text, I would advocate mnemonic
ascii as the downgrading mechanism; this makes the messages
intelligble at a 7-bit site. Actually this also makes sense for
text with only a small amount of 8-bit characters.
Keld