Bug#806717: kmail: Please do not fix headers of filtered multipart messages. It breaks PGP signatures.

[Mikko Tuumanen]

A workaround for pgp signed messages in the bogofilter case:
Settings -> Configure Filters -> Bogofilter Check -> Add Anywhere in 
Headers, does not contain, application/pgp-signature

This way signed messaged are not filtered and not broken.