* Default Content-Type to text/plain for not multipart messages
Parse body of messages without subparts as text/plain if Content-Type
header is not set.
* narrow check for defaulting to `text/plain`
take message body as `text/plain` only if message has only one part
and has no `Content-Type` header
Co-authored-by: Clément DOUIN <soywod@users.noreply.github.com>
This brings it in line with how notify-query works and how the wiki
defines it to also be a global option.
Co-authored-by: Clément DOUIN <soywod@users.noreply.github.com>
