I've never used Slack because last I remember they don't allow local installation, therefore no access to any documents that were dropped in, is this still the

I've never used Slack because last I remember they don't allow local installation, therefore no access to any documents that were dropped in, is this still the case? I have been using mattermost instead, and wrote a bot which processes all documents uploaded into mattermost, extracts metadata, creates tags and summaries for each, archives the documents in an ECM -categorizing them by their tags, and opens them up for full text solr search. My understanding is that a custom solution like this wouldn't be possible with Slack, or would otherwise require more hacky solutions?