var downLoadImgUrl = $("#QRCode-item canvas")[0].toDataURL("image/png");

var _fixType = function(type) {
    type = type.toLowerCase().replace(/jpg/i, ‘jpeg‘);
    var r = type.match(/png|jpeg|bmp|gif/)[0];
    return ‘image/‘ + r;
};
var type = ‘png‘;
var fixType = _fixType(type);
downLoadImgUrl = $("#QRCode-item canvas")[0].toDataURL(fixType);
downLoadImgUrl = downLoadImgUrl.replace(fixType, ‘image/octet-stream‘);