js如何删除input属性为file的值

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); 
 }  
}