mm-body-charset-encoding-alist

mm-body-charset-encoding-alist is a variable defined in `mm-bodies.el'.
Its value is

((iso-2022-jp . 7bit)
 (iso-2022-jp-2 . 7bit)
 (utf-16 . base64)
 (utf-16be . base64)
 (utf-16le . base64))

Documentation:
Alist of MIME charsets to encodings.
Valid encodings are `7bit', `8bit', `quoted-printable' and `base64'.

You can customize this variable.