OK, I've fixed it.
1. First of all the zip should be called FileManager.zip
2. The empty file should be a .jar file not a .java file.
It's a nice example of how to create pure JavaScript plugins. Well done Dimon :)
Try it now.
Dimon: If you want to add images and resources, make sure they go at the top level of the zip, anything at a lower level folder will be discarded when building an APK (good for docs though). All assets at the top level folder will be copied into the 'Plugins' private folder and can be accessed at run time.
Here is a snippet from the micro:bit plugin that shows how to access images from your plugin at run time.
var plugDir = app.GetPrivateFolder( "Plugins" );
var img = app.CreateImage( plugDir+"/microbit/micro-bit.png", sz );
Regards
David