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This class represents a file or form item that was received within a multipart/form-data POST request. 
multipart/form-data POST是啥意思啊？

------解决方案--------------------
你要是上传文件的话,必须在form中加enctype="multipart/form-data" 才可以.提示就是说的这件事情.把你的Form改成如下所示:
<form name="form1" enctype="multipart/form-data" action="aaa.action" method="post">
------解决方案--------------------
你要做文件上传的话必须这样写<form enctype="multipart/form-data" action="****" method="post">而且method必须是post，enctype="multipart/form-data"是以二进制流的形式提交表单