[qmf] Use EightBit for text and email files. Contributes to JB#12571
Previously attachments with text/* or message/rfc822 encoding were base64 encoded for no good reason. Change them to use EightBit transfer encoding.
This fixes sending .eml files as an attachment that previously was totally broken. Additionally it changes behaviour for text type files which no longer need to be base64 encoded using 25% less data.