This is not currently possible in a nanoflow. You will have to do this in a microflow instead. It may have something to do with the fact that a nanoflow runs on the client and the microflow on the server.
You can do the download of the file in a microflow and call the microflow from your nanoflow. What is your use case for this just out of interest? I see you mention native. I would check whether a connection to the server is available and if it is then call it in a microflow from your nanoflow.