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" οnclick="cleanFile('file1')" />