Upload File and Attach to Ticket

I'm using /api/upload to upload files, and using the response to submit attachments on a new ticket (api/tickets). The upload works, the ticket gets created, and the attachment is present in the dashboard, but the image url's return a 403, and appear as a broken img in the dashboard. I also included public: true inside the attachment object with no luck - I think true is the default anyway according to the docs. Any ideas?