springMVC过滤器过滤文件上传，获取参数为空 400 请求报错 

使用springMVC，图片上传，过滤器拦截获取不了请求参数。
页面发起的请求有三个参数：op，name，picture。
HTML代码如下：

<form action="<%=request.getContextPath()%>/app"  method="post" enctype="multipart/form-data"> op:<input name="op" value="user.regist" /><br/> 姓名(name):<input name="name" /><br/> 头像(picture):<input name="picture" type="file"><br/> <input type="submit"/> </form>

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletRequest httpRequest = (HttpServletRequest)request; String op = httpRequest.getParameter("op"); op = op.replace(".", "/"); request.getRequestDispatcher("/app/"+op).forward(request, response); }