求教Ajax结合struts上传的问题（单文件无刷新）-- file关联不上来

[macower]

  $.ajax({
  							    url:"analyModel!uploadFile.action",
  							    context: document.body,
  							    dataType:"json", 
  							  	contentType:"multipart/form-data",
  							    type:"post",							   
  							    success:function(data){
  							       var d  = eval(data);
  							       alert(d.message);
  							    }
  							  });	

private File file;
private String fileFileName;
private String fileContentType;

在方法中file为空，请有经验哥们救救.
(最好是注解)