I have used Slack for notifications for a long time, but started getting annoyed at the limitations of the free accounts. Mattermost was the closest I could find to a self-hosted alternative.
I built an integration to take advantage of Mattermost’s formatting, while trying to keep it as compatible with notify.slack as I could. Hopefully someone finds it useful, there are some older threads here using REST commands and this seems a bit cleaner.
And some obligatory screenshots (from mobile and desktop apps).
