Fix #523: Strip all non text/plain parts in multipart/alternative if keywords are present
Strip all non text/plain parts in multipart/alternative if keywords are present to stop leaking them, even when it is encapsulated in multipart/mixed. This makes sure that emails with the following structure don't leak keywords:
multipart/mixed
'- multipart/alternative
|- text/plain
'- multipart/related
|- text/html
'- image/png
This kind of structure is generated by Thunderbird 102.3.0 when images are included in the email. Other email clients might do that as well.
See also:
https://www.rfc-editor.org/rfc/rfc2046#section-5.1.3
https://www.rfc-editor.org/rfc/rfc2046#section-5.1.4
https://www.rfc-editor.org/rfc/rfc2387
fixes #523 (closed)