mm-inlined-types

mm-inlined-types is a variable defined in `mm-decode.el'.
Its value is

("image/.*" "text/.*" "message/delivery-status" "message/rfc822" "message/partial" "message/external-body" "application/emacs-lisp" "application/x-emacs-lisp" "application/pgp-signature" "application/x-pkcs7-signature" "application/pkcs7-signature" "application/x-pkcs7-mime" "application/pkcs7-mime" "application/x-gtar-compressed" "application/x-tar" "application/zip" "application/pgp")

Documentation:
List of media types that are to be displayed inline.
See also `mm-inline-media-tests', which says how to display a media
type inline.

You can customize this variable.