Report it. In light of this information, no one is gonna give a damn about you poking around in Slack; if the sysadmins have been doing their job then you poking around this wouldn't be a cause for concern as you should have been denied access to anything that wasnt in your domain so to speak.
If anything, it will look worse on you if you don't report it, as it raises the possibility that you may have been attempting to exploit the information for your own gain. At the very least it will raise questions as to why you were accessing this file without raising alarms. In other words you will no longer look like an innocent bystander.