Use base64 encoding for all non-text attachments

KMime::encodingForData() uses character occurence frequency to determine
which encoding to use. This does not work for binary attachments because
any characters can appear there. If a wrong encoding is chosen, CRLF/LF
transformations can break the attachment.

Differential Revision: https://phabricator.kde.org/D5737
BUG: 95733
FIXED-IN: 5.5.1