Retrieve the binary file associated with an Attachment. Authenticate the API request with an API key or an OAuth token with read_attachments. The API returns a 302 redirect; follow the Location header to retrieve the file bytes from storage. The redirect target may be on another host: do not forward your ServiceM8 API credentials to it. Treat the returned URL as temporary and request this endpoint again for a fresh URL when needed. The file response uses the stored file's content type, rather than the Attachment JSON metadata schema.