Matrix has bridges to IRC (freenode, moznet, oftc, snoonet etc), Slack, Gitter, Rocket.Chat, XMPP, SMS, SIP and others. The point is to 'matrix' all the different networks out there into one single decentralised network.
End-to-end encryption is being discussed, but as of yet it has not been implemented. Messages currently are only protected by TLS in transit. In Matrix's current state it's possible for anyone to join what are meant to be private discussion rooms and servers can read all messages.
With Docker set up, you can install Mattermost with the following command: docker run --name mattermost-dev -d --publish 8065:80 mattermost/platform For other cases, installation instructions can be found here.
Sending anything besides text is not easy or intuitive and depends on the client. You can't send files to a channel and sending files between two users requires making sure that both have dcc (or xdcc for larger files) set up and enabled in their respective clients. There are no link previ...