Rocketchat looks rather good as a Slack replacement! Open source, data on our own server. It has an import function for Slack exports so now that we have set up our own rocket chat server (named slack to make the switch easy on the mind), I’m importing all our Slack content over. Much better than staying with Slack which at our current usage level runs at about 75 Euro / month, whereas our managed hosting solution is 12 Euro / month including a 1 hr SLA response time. It integrates with our cloud, allowing direct drag and drop of any files.