Description:

Right now Rocket.Chat´s Slack Import only imports public channels, users and messages.
The coporate-export of Slack gives the opportunity to also export:

• private channels ("groups")(groups.json)
• direct messages (dms.json)
• multi-user-conversations (mpims.json)

The importer.js is processing all the message-subfolders but only takes care about the public channels, described in channels.json.

(To me) it looks like the routines are already there to process the other json-files mentioned above, especially because the importer already handles all the message-subfolders. So it would be great if this could be implemented.

Steps to reproduce:

1. Go to AdminPanel->Import->Slack
2. Select Sack-Coporate-Export-Zip-file
3. Wait for Rocket-Chat Importer to process the file
4. See that all users, all messages but only public channels are importable

Expected behavior:

Also private channels, direct messages and multi-user-conversations should be importable

Server Setup Information:

• Version of Rocket.Chat Server: latest (0.74.3)
• Operating System: Synology DSM
• Deployment Method: Docker Container
• Number of Running Instances: 1
• NodeJS Version: 8.11.4
• MongoDB Version: 4.0.5

I would be happy to provide more information or a testfile if nessecary.