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.
Image data probably should just go straight to base64, but there are cases
of image formats that might use quoted-printable to advantage (remember, it is
more efficient for alphabetic data). Anything is possible.
A gateway that is choosing an appropriate encoding without full knowledge of
all types might do well to use a combination of the type, an analysis of
the data, and some heuristics to encode things. No other approach seems
appropriate.
Ned