I have a working custom designed chat application using REST ECE Messaging APIs. Now I want to extend this to allow agent's to send attachments to customers. When an agent send's an attachment, as expected, I receive a callback on my apps webbook URL with type "uploadAttachment". The message has the id, fileName, contentType, size and messageText attributes. There is no content URL field sent within this message. How can my custom chat app get the actual attachment contents ?