The WSS BNS (Block Notification Service) provides a popup that includes the name of the file that was blocked.
CloudSoc Gateway was set to obfuscate activity logs for external users, this is working as designed for external users. If you see this occur for an Internal user, check the investigate upload activity in CloudSOC and see if the user is listed as external or internal.
An issue was found with the Dropbox Gatelet where the SaaS user was not recognized. The user is reported as an internal user instead of an external user. Because of this issue the BNS reported the filename in the Block notification Service, instead of the filename being reported as unknown.
If a customer has a policy that acts upon external users differently than internal user this would be a more serious issue. Broadcom is aware of the issue and is planning a fix in the near future.
iCloud gatelet is also suspected of behaving similarly and is currently being investigated.