gnus-inhibit-mime-unbuttonizing

gnus-inhibit-mime-unbuttonizing is a variable defined in `gnus-art.el'.
Its value is

nil

Documentation:
If non-nil, all MIME parts get buttons.
When nil (the default value), then some MIME parts do not get buttons,
as described by the variables `gnus-buttonized-mime-types' and
`gnus-unbuttonized-mime-types'.

You can customize this variable.

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