how to modify mailcap-mime-data and friends

[Michal]

Hallo Group Members

I would like to change external viewers my gnus uses. I am guessing
that I need to edit mailcap-mime-data. Unfortunatelly I cannot
customize it (M-x customize-variable: mailcap-mime-data does not
work). I can of course act manually on the list but this is probably
not the way You do it. Can You tell me how to do it in neat way?

Regards,
Michal