Use base64 encoding for all non-text attachments, only use KMime::encodingForData()
for text/* and message/* attachments.
KMime::encodingForData() uses character occurrence frequency to choose optimal encoding,
but in case of binary documents (like PDF) this can get completely off and choose the wrong
encoding and CRLF/LF conversion then corrupts the binary data, making the attachment unreadable.
See https://bugs.kde.org/show_bug.cgi?id=95733 for details