上传文件后alert的值是undefined 取不到文件怎么解决

<input type="file" multiple="multiple" id="sys_attachment_span"  name="sys_attachment_span"/>

<input type="button"  onclick="uploadf()" value="上传">


function uploadf(){
        //var fileInput = document.getElementById("sys_attachment_span");
        
        var filess = document.getElementById("sys_attachment_span").files;
        
        alert("1:"+filess);
        if(filess.length == 0){
            alert('请选择文件！');
            return;
        }


}