I have looked in to the following template by @Matt Burgess, which works fine for storing the data on my machine. 
https://gist.github.com/mattyb149/6c09608b09ea07639e1a
I have tried to save  json file from https://thingspeak.com/channels/64519/feed.json.
From InvokeHTTP ---> PutFile is not receiving any data. 
I am new to NIFI, i might be missing something.