Return the content-transfer-encoding used for body encoding.
This is either the string 'quoted-printable' or 'base64' depending on
the encoding used, or it is a function in which case you should call
the function with a single argument, the Message object being
encoded. The function should then set the Content-Transfer-Encoding
header itself to whatever is appropriate.
Returns "quoted-printable" if self.body_encoding is QP.
Returns "base64" if self.body_encoding is BASE64.
Returns conversion function otherwise.