mm-automatic-display

mm-automatic-display is a variable defined in `mm-decode.el'.
Its value is

("text/plain" "text/enriched" "text/richtext" "text/html" "text/x-verbatim" "text/x-vcard" "image/.*" "message/delivery-status" "multipart/.*" "message/rfc822" "text/x-patch" "text/dns" "application/pgp-signature" "application/emacs-lisp" "application/x-emacs-lisp" "application/x-pkcs7-signature" "application/pkcs7-signature" "application/x-pkcs7-mime" "application/pkcs7-mime" "application/pgp\\'" "text/x-org")

Documentation:
A list of MIME types to be displayed automatically.

You can customize this variable.