HTML的input标签为file类型时按钮中显示上传文件的名字

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主要html代码：

<a href="javascript:;" class="a-upload">
    <div id="shangchuan" onchange="successload()">
    </div>
//使用onchange而非onclick，onclick会在弹出框出现之前，而onchange会在选择完文件后
    <script>
    var filename = document.getElementById('file');
    function getfilename(){
        if(filename.files[0] == undefined){  
                alert('未上传文件！');  
        }
        else{  
            //alert('上传成功！');           
            document.getElementById('shangchuan').innerHTML=filename.files[0].name;
        }  
    }
    </script> 

css样式采用了：
http://www.haorooms.com/post/css_input_uploadmh

.a-upload {
    padding: 4px 10px;
    height: 20px;
    line-height: 20px;
    position: relative;
    cursor: pointer;
    color: #888;
    background: #fafafa;
    border: 1px solid #ddd;
    border-radius: 4px;
    overflow: hidden;
    display: inline-block;
    *display: inline;
    *zoom: 1
}

.a-upload  input {
    position: absolute;
    font-size: 100px;
    right: 0;
    top: 0;
    opacity: 0;
    filter: alpha(opacity=0);
    cursor: pointer
}

.a-upload:hover {
    color: #444;
    background: #eee;
    border-color: #ccc;
    text-decoration: none
}