Noticed during testing of filename encodings.
In Multipart signed messages (in contrast to multipart encrypted) Outlook
rewrites the filenames in the Resulting Mail. If we signed a different filename
representation the signature will brake.

This currently happens at least for Quoted Printable encoded filenames in higher
UTF8 values. ? If we sign and write in our MIME structure the filename exactly
as encoded as Outlook would send it in an unsigned mail Outlook replaces the
character with ?? and does not use QP encoding. We should do something similar
or provide a warning that such filenames can break signatures.