HelpDesk allows your agents to make the particular attachments invisible in issue view in the Customer Area (still visible via Jira). It can be convenient to share some internal files while working on an issue that is available to a customer.
The functionality is disabled by default. To enable it, navigate to HelpDesk Administration → Customer Area → General Setting → Miscellaneous and tick the relevant checkbox:
The 'cog' icon will be enabled for the issue attachments. By clicking on it you can specify which groups will be able to see the attachment via issue view in the Customer Area. Files with some restrictions are marked as locked.