mm-discouraged-alternatives

mm-discouraged-alternatives is a variable defined in `mm-decode.el'.
Its value is

nil

Documentation:
List of MIME types that are discouraged when viewing multipart/alternative.
Viewing agents are supposed to view the last possible part of a message,
as that is supposed to be the richest. However, users may prefer other
types instead, and this list says what types are most unwanted. If,
for instance, text/html parts are very unwanted, and text/richtext are
somewhat unwanted, then the value of this variable should be set
to:

("text/html" "text/richtext")

Adding "image/.*" might also be useful. Spammers use it as the
preferred part of multipart/alternative messages. See also
`gnus-buttonized-mime-types', to which adding "multipart/alternative"
enables you to choose manually one of two types those mails include.

You can customize this variable.