RFE: Add MessageIsBinary protection

[spaetz]

trafficstars

We have a MessageIsVoiceProtection and a MessageIsMediaProtection, which is great. However, I moderator a room in which I would like to prohibit the posting of binaries (think malware). Previously, I had written a custom bot that redacted all messages that contained attachements whose type was not whitelisted. It would be awesome if draupnir allowed that!

Two possible solutions:

• Implement MessageIsBinaryProtection would immediately solve my problem, but it could mean that we need dozens of possible Protections for people wanting to prohibit specific content.
• Implement MessageIsForbiddenAttachment (or similar) which allows a Black/Whitelist of attachments to be configured.

Thanks for considering, and a ton of thanks for maintaining draupnir, it is super useful!