It won't be ever possible to read/modify any file through Lua scripts, because it is a top 1 security flaw.
If you want to share a map, use "Copy Json" / "Share online" / "Add from Json" / "Add from URL" buttons in the map configuration menu.
More convenient ways of sharing maps and mods will be available as soon as it is safe for the players to run 3rd party scripts - you can help with that by reviewing the list of available methods in assets/res/luajava-map-generator-report.html and reporting the dangerous ones (those that can be used to abuse player's device or ruin his progress) right here on the tracker.