经测试代码如下：
<form name="form5" id="form5" method="post" action="#">
<input type="file" name="file5" id="file5" onchange="preview5()"/>
</form>
<script type="text/javascript">
/**
* 预览待上传的本地图片
*
* @param
* @arrange (512.笔记) jb51.cc
**/
function preview5(){
var x = document.getElementById("file5");
if(!x || !x.value) return;
var patn = /\.jpg$|\.jpeg$|\.gif$/i;
if(patn.test(x.value)){
var y = document.getElementById("img5");
if(y){
y.src = "file://localhost/" + x.value;
}else{
var img=document.createElement("img"); img.setAttribute("src","file://localhost/"+x.value);
img.setAttribute("width","120");
img.setAttribute("height","90");
img.setAttribute("id","img5");
document.getElementById("form5").appendChild(img);
}
}else{
alert("您选择的不是图像文件。");
}}
</script>
// 来自：编程之家 jb51.cc（jb51.cc） 