Download one decoded attachment

Requires a full key or an inbox key (bound inbox only). index is the attachment position from the message, starting at zero. A negative index is an invalid request. The response content type is the attachment type. Content-Disposition is attachment with a safe filename. X-Content-Type-Options is nosniff and Content-Security-Policy is sandbox, including for HTML. A full raw-read limit returns 503. Deleted or expired messages are not found.