file association for mac! associating file is done automatically when you move the app in your system but there is also a button to set it manually i had to disable argument parsing and to subclass qapplication to make this work please test

#include <QtGui/QApplication>
#include "mainwindow.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}