Calls a system dialog to allow the user to choose a file
For audio files:-
app.ChooseFile("Choose a file", "audio/*", assets_onFileChoose);
For text files:-
app.ChooseFile("Choose a file", "text/*", onFileChoose);
For image files:-
app.ChooseFile("Choose a file", "image/*", onFileChoose);
For all files:-
app.ChooseFile("Choose a file", "*/*", onFileChoose);