It's strange that the file dialog for the nw.js plugin doesn't work, right?
NW.js is for desktop, it doesn't work on mobile.
With Cordova plugin you have two options:
1. Find someone who can make it into a C3 addon.
2. Export your game as Cordova project and manually add the Cordova plugin using Cordova CLI, Android Studio or XCode. You will need to call plugin methods with Javascript, but this isn't very hard. For example, to open the file dialogue:
fileChooser.open(successCallback, failureCallback);