Who has permissions to see a preview of a file shared from a cloud storage service?
If a file is public to everyone in your organization, coworkers who have enabled the cloud storage integration for their Workplace account will see a rich preview and/or the files' title. If the file isn't public, they'll see a prompt to connect their accounts to determine if they have permissions to view the title and/or a preview from within Workplace.
Workplace will only show a preview and/or title to those who have permission to view the file. If a coworker hasn't enabled the integration, they'll have the choice to enable the integration and see the previews.
Note: Content integration file previews don't work in multi-company groups. The files will still appear as URL links.
Keep in mind that if a file is encrypted/password protected, no preview will be shown.