make smime.p7n readable

[Hubhubhurra]

Hallo,
some email-clients merge a digital sign public certificate together with the message-body to a single zip which snappy only show as attachment smime.p7n. No text is readable. To read it is necessary download the attachment and unzip it.
Thunderbird and other client-programms can show the message directly and it was a great think if snappy can this too.

with regards

[the-djmaze]

Now you can https://github.com/the-djmaze/snappymail/releases/tag/v2.35.0