#include <QCoreApplication>
#include <QString>
#include <QUrl>
#include <QDir>
#include <QDesktopServices>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    QString file = QDir::toNativeSeparators("C:/Users/eroeoes/Pictures/rk.png");
    QString mailto = "mailto:user@foo.com?subject=Just a test&attachment=\"" + file + "\"";
    QDesktopServices::openUrl(QUrl(mailto));
    
    return a.exec();
}