Ability to add an email attachment file extension black/whitelist

[patch0]

Would be nice to be able to block certain attachments to email, in addition to the usual antivirus scanning

Originally reported on Bytemark's Gitlab by @patch0 on 2016-08-10T10:31:40.021Z

[patch0]

I've looked at clamav and spamassassin briefly and I can't see a way to do it by extension per-se, but maybe mime-type would be a better match anyway.