QString file =QFileDialog::getSaveFileName(0,tr("choose a filename to save under"), QDir::currentPath(),"Images(*.bmp)");
                     if (image.isNull()){
        QMessageBox::information(0, tr("Image SaveAs"),
            tr("Cannot save to %1").arg(file));
        ShowMessage("Null Image.");
        return false;
    }else {
        ShowMessage(tr("save to %1 succeed").arg(file));
        return image.save(file,"BMP");
如何才能使保存的文件自动带上格式而不需要手动输入