清空file值 各种浏览器支持

 <script>

function cleanFile(id){

    var _file = document.getElementById(id);

    if(_file.files){

        _file.value = "";

    }else{

        if (typeof _file != "object"){ return null; }

        var _span = document.createElement("span");

        _span.id = "__tt__";

        _file.parentNode.insertBefore(_span,_file);

        var tf = document.createElement("form");

        tf.appendChild(_file);

        document.getElementsByTagName("body")[0].appendChild(tf);

        tf.reset();

        _span.parentNode.insertBefore(_file,_span);

        _span.parentNode.removeChild(_span);

        _span = null;

        tf.parentNode.removeChild(tf);

    }

}

</script>

<input id="file1" type="file" />

<input type="button" value="清空" onclick="cleanFile('file1')" />