file样式设置
.upload_bg{position:relative; text-align:center; }
    .upload_bg input {position:absolute; left:0px;*left:-10px; top:0px; height:19px; width:100%;*width:1px; filter:alpha(opacity=0);opacity:0.0;}

<input type="file" size='1' style="width:100px">
加上size="1"即可。