图片上传预览问题

<html> 
    <head> 
        <title>get file input full path</title> 
    <script language='javascript'> 
        function getFullPath(obj) 
        { 
            if(obj) 
            { 
                //ie 
                if (window.navigator.userAgent.indexOf("MSIE")>=1) 
                { 
                    obj.select(); 
                    return document.selection.createRange().text; 
                } 
                //firefox 
                else if(window.navigator.userAgent.indexOf("Firefox")>=1) 
                { 
                    if(obj.files) 
                    { 
                        return obj.files.item(0).getAsDataURL(); 
                    } 
                    return obj.value; 
                } 
                return obj.value; 
            } 
        } 
    </script> 
    </head> 
    <body> 
        <input type="file" onChange="document.getElementById('img').src=getFullPath(this);" /> 
    <img id="img" height="174" width="234"/> 
    </body> 
</html>这个上传预览兼容IE6.FF.IE8.我把他放在aspx页面下IE8无效果.FF.IE6可以.请问高手们这是怎么回事？