We use Keybase for work chat. It's quite good, and the file sharing options make it even better--you get a synthetic filesystem mounted under /keybase, so you can share a file to the whole team by copying it to /keybase/team/TeamName/, or share it with just one person by copying to /keybase/private/<you>,<them>/

Has to be on-prem if the networks are air-gapped. KB Teams also has a long way to go to meet feature-parity - channels, integrations, etc.