Feature: Expand email builder API to support forced content Content-Transfer-Encoding for attachments, like quoted-printable, base64, 7BIT and others

#399 added support for Content-Transfer-Encoding on body content level (so plain text, HTML text and iCalender), but we can go further and enforce transfer encoding for attachments as well. This way we can make sure plain text is stored as base64 in the resulting EML for example.