Unsure what return does for QQ Uploader (errors in ie9)

fileupload.js

 _isValidFileDrag: function(e){
        var dt = e.dataTransfer,
            // do not check dt.types.contains in webkit, because it crashes safari 4
            isWebkit = navigator.userAgent.indexOf("AppleWebKit") > -1;
        console.log(dt);

        // dt.effectAllowed is none in Safari 5
        // dt.types.contains check is for firefox
        return dt && dt.effectAllowed != 'none' &&
            (dt.files || (!isWebkit && dt.types.contains && dt.types.contains('Files')));
    }

// errors in ie9 undefined method at line with return character 9