后台：
ActionContext context = ActionContext.getContext();
HttpServletResponse response = (HttpServletResponse) context.get(ServletActionContext.HTTP_RESPONSE);
response.setCharacterEncoding("UTF-8");
HttpServletRequest request = (HttpServletRequest) context.get(ServletActionContext.HTTP_REQUEST);
request.setCharacterEncoding("UTF-8");
response.getWriter().print("{success:false,msg:'上传失败,文件格式错误'}");
前台：
failure: function(form, action){
Ext.MessageBox.updateProgress(1);
Ext.MessageBox.hide();
if(action.result.msg){
Ext.Msg.alert('错误', action.result.msg );
}else{
Ext.Msg.alert('错误', '文件上传失败');
}
}
这时弹出的窗口是乱码，前后台已经都是UTF-8，在后台print之前增加一句代码解决问题：response.setContentType("text/html; charset=utf-8");