iframe实现文件的异步提交

<form action="upload.jsp" id="form1" name="form1" encType="multipart/form-data"  method="post" target="hidden_frame" >  

　　<input type="file" id="file" name="file" style="width:450">  

　　<INPUT type="submit" value="上传文件"><span id="msg"></span>  <br>             

　　<iframe name='hidden_frame' id="hidden_frame" style='display:none'></iframe>

</form>