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Mattermost is an open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams. Wikipedia
Desktop Client
Download: https://mattermost.com/download/#mattermostApps
If you have trouble to connect to your Mattermost server on Windows, e.g. through proxy, use the MSI installer instead of the EXE:
(thnx rudi)