Browser downloads files as attachments

@WebServlet("/downloadServlet")
public class DownloadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. Get request parameters , File name 
        String filename = request.getParameter("filename");
        //2. Use byte input stream to load file into memory 
        //2.1 Find the file server path 
        ServletContext servletContext = this.getServletContext();
        String realPath = servletContext.getRealPath("/img/" + filename);
        //2.2 Associate with a byte stream 
        FileInputStream fis = new FileInputStream(realPath);

        //3. Set up response Response header for 
        //3.1 Set response header type ：content-type
        String mimeType = servletContext.getMimeType(filename);// retrievable mime type 
        response.setHeader("content-type",mimeType);
        //3.2 Set the opening mode of response header :content-disposition

        // Solve the problem of Chinese file name 
        //1. obtain user-agent Request header 、
        String agent = request.getHeader("user-agent");
        //2. Use the tool class method to encode the file name 
        filename = getFileName(agent, filename);

        response.setHeader("content-disposition","attachment;filename="+filename);
        //4. Write the data from the input stream into the output stream 
        ServletOutputStream sos = response.getOutputStream();
        byte[] bytes= new byte[1024 * 8];
        int len = 0;
        while((len = fis.read(bytes)) != -1){
            sos.write(bytes,0,len);
        }

        fis.close();


    }

// Get the Chinese file name of different browser attachments

 public String getFileName(String agent, String filename) throws UnsupportedEncodingException {
        if (agent.contains("MSIE")) {
            // IE browser 
            filename = URLEncoder.encode(filename, "utf-8");
            filename = filename.replace("+", " ");
        } else if (agent.contains("Firefox")) {
            //  Firefox 
            BASE64Encoder base64Encoder = new BASE64Encoder();
            filename = "=?utf-8?B?" + base64Encoder.encode(filename.getBytes("utf-8")) + "?=";
        } else {
            //  Other browsers 
            filename = URLEncoder.encode(filename, "utf-8");
        }
        return filename;
    }