JS判断是否为图片后缀名

// 增加一个名为 IsPicture 的函数作为
    // String 构造函数的原型对象的一个方法。
    String.prototype.IsPicture = function()
    {
        //判断是否是图片 - strFilter必须是小写列举
        var strFilter=".jpeg|.gif|.jpg|.png|.bmp|.pic|"
        if(this.indexOf(".")>-1)
        {
            var p = this.lastIndexOf(".");
            //alert(p);
            //alert(this.length);
            var strPostfix=this.substring(p,this.length) + '|';        
            strPostfix = strPostfix.toLowerCase();
            //alert(strPostfix);
            if(strFilter.indexOf(strPostfix)>-1)
            {
                //alert("True");
                return true;
            }
        }        
        //alert('False');
        return false;            
    }

//<!--前台HTML文件//-->//

//预览图片
        function showImg()
        { 
            document.getElementById("imgPic").removeAttribute("height");
            if(document.getElementById("spnPic").style.display!="none")document.getElementById("spnPic").style.display='none';
            if(document.getElementById("Image").value.trim()!='')
            {    
                //IsPic();
                document.getElementById("imgPic").style.display='';
                document.getElementById("imgPic").src=document.getElementById("Image").value.trim();    
                if(document.getElementById("imgPic").height>150) document.getElementById("imgPic").height=150;
                //if(document.getElementById("imgPic")>300) document.getElementById("imgPic").width=300;            
            }            
            else
            {
                document.getElementById("imgPic").style.display='none';
                document.getElementById("imgPic").src='';
            }        
        }
        
        //图片验证
        function IsPic()
        {
            if(document.getElementById("Image").value.trim()!='')
            {
                if(document.getElementById("Image").value.IsPicture()==false)
                {
                    alert('请选择正确的图片路径！');
                    document.getElementById("Image").focus();
                    document.getElementById("Image").select();
                }
            }
        }