java实现下载 设置响应头的mimetype类型问题

Question

刚学到如何实现下载 课件上写要设置两个响应头 其中一个是

ServletContext sc = getServletContext();
String mimeType = sc.getMimeType(filename);
response.setContentType(mimeType);

目的是为了告诉浏览器文件类型
但是当我尝试这么做的时候会出现这个警告

并且将这段代码注释掉依然能以正常的格式下载，并且警告也没有了

response.setContentType(mimeType);

请问下为什么会出现这两种情况
以下为完整代码 idea tomcat8 jdk8 chrome

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;

@WebServlet("/downloadServlet")
public class DownloadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        response.setContentType("text/html;charset=utf-8");
        String filename = request.getParameter("filename");
        ServletContext sc = getServletContext();
        String mimeType = sc.getMimeType(filename);
        String realPath = sc.getRealPath("/WEB-INF/" + filename);


        File f = new File(realPath);

        if(f.exists()){
            response.setHeader("content-disposition","attachment;filename="+ URLEncoder.encode(filename,"utf-8"));
            response.setContentType(mimeType);
            FileInputStream fis = new FileInputStream(f);
            ServletOutputStream os = response.getOutputStream();
            byte[] b = new byte[1024 * 8];
            int len;
            while ((len=fis.read(b))!=-1){
                os.write(b,0,len);
            }
            os.close();
            fis.close();
        }

    }
}

Answer 1

目前的一些浏览器已经支持不写也能正常下载，不过最好写下