we remove signatures but don't change 'Content-Type'
Some clients give errors to find a 'multipart/signed' content type but now 'signature' part.
(from redmine: created on 2016-03-24, relates #7734 (closed))
Some clients give errors to find a 'multipart/signed' content type but now 'signature' part.
(from redmine: created on 2016-03-24, relates #7734 (closed))