Hey Shin, you have a couple options depending on your use case:
1.
Videos uploaded as attachments in Slack are synced to the companion website. By default, the website is public but you can configure it to require a login. The login would be through OAuth into Slack.
2.
You can upload files today via our admin REST API. However, at the moment all files are public. If you want this option to be private files, could you explain more about your use case?