It's fair to say that the Steam SDK is rather comprehensive (in a voluminous sort of way)... After a bit of trial and error, a couple of beers and some swearing, I have achieved upload and download success using — 's awesome plugin. Pretty sweet using nw.js v12 in steam - cloud save, achievements, overlay all working in windows. The next couple of days will be taken up by doing battle with linux and osx. I have no idea what I'm doing on those platforms, while my mac is over 5 years old and hates nw.js, lol....
I'm excited about nw.js v13, but I understand that all greenworks plugins will need to be adapted for it... :/