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.