gnus-buttonized-mime-types is a variable defined in `gnus-art.el'.
Its value is

List of MIME types that should be given buttons when rendered inline.
If set, this variable overrides `gnus-unbuttonized-mime-types'.
To see e.g. security buttons you could set this to
`("multipart/signed")'. You could also add "multipart/alternative" to
this list to display radio buttons that allow you to choose one of two
media types those mails include. See also `mm-discouraged-alternatives'.
This variable is only used when `gnus-inhibit-mime-unbuttonizing' is nil.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.