Attachment needs to know the range of bytes where the body part of the
attachment is located. The Attachment.raw field contains the entire
attachment, headers and body. The new Attachment.body fields contains a
`StrBuilder` which contains the offset and length of the body part inside
`raw`.
Check subattachments in has_attachments check.
Instead of getting a flattened attachment view of multipart/mixed (eg
[multipart/mixed, text/plain, text/plain]) get only the subattachments
(eg [text/plain, text/plain]). Don't count text-only multipart/mixed as attachments