[evolution-patches] evolution certificate dialog not filtering by mimetype

[Gilles Dartiguelongue <dartigug esiee fr>]

Hi list,

following bug #396645 reporting that certificate import dialogs aren't
filtering by mimetype, I've cooked and attached a second patch that
finally works and present the user only the certificate (s)he can import
by default.

Please review/comment.
-- 
Gilles Dartiguelongue <dartigug esiee fr>