怎样使用trigger方式实现不用点击file类型的input弹出文件选择对话框

var button1 = document.getElementById("button1");  
    var button2 = document.getElementById("button2");  
    button1.onclick = function(){alert("button1");trigger(button2,"onclick");}  
    button2.onclick = function(){alert("button2");}  
    function trigger(ele,event)   
    {ele[event]();}

<div class="ps-image" style="width:300px;height:300px;border:0px sold red; background:url(&#39;/images/ps.png&#39;)">  
    <input type="file" id="file" style="filter:alpha(opacity=0);opacity:0;width:100%;height:100%;"/>  
    </div>