mm-attachment-override-types

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

("text/x-vcard" "application/pkcs7-mime" "application/x-pkcs7-mime" "application/pkcs7-signature" "application/x-pkcs7-signature")

Documentation:
Types to have "attachment" ignored if they can be displayed inline.

You can customize this variable.