mm-content-transfer-encoding-defaults

mm-content-transfer-encoding-defaults is a variable defined in `mm-encode.el'.
Its value is

(("text/x-patch" 8bit)
 ("text/.*" qp-or-base64)
 ("message/rfc822" 8bit)
 ("application/emacs-lisp" qp-or-base64)
 ("application/x-emacs-lisp" qp-or-base64)
 ("application/x-patch" qp-or-base64)
 (".*" base64))

Documentation:
Alist of regexps that match MIME types and their encodings.
If the encoding is `qp-or-base64', then either quoted-printable
or base64 will be used, depending on what is more efficient.

This list is only consulted when encoding MIME parts in the
bodies -- not for the regular non-MIME-ish messages.

You can customize this variable.