Servlet redirect to a file download with name changed

public class FileServlet extends HttpServlet {
    String getContentType(String fileName) {
        String extension[] = { // File Extensions
            "txt", //0 - plain text
            "htm", //1 - hypertext
            "jpg", //2 - JPEG image
            "png", //2 - JPEG image
            "gif", //3 - gif image
            "pdf", //4 - adobe pdf
            "doc", //5 - Microsoft Word
            "docx",
        }; // you can add more
        String mimeType[] = { // mime types
            "text/plain", //0 - plain text
            "text/html", //1 - hypertext
            "image/jpg", //2 - image
            "image/jpg", //2 - image
            "image/gif", //3 - image
            "application/pdf", //4 - Adobe pdf
            "application/msword", //5 - Microsoft Word
            "application/msword", //5 - Microsoft Word
        }, // you can add more
                contentType = "text/html";    // default type
        // dot + file extension
        int dotPosition = fileName.lastIndexOf('.');
        // get file extension
        String fileExtension =
                fileName.substring(dotPosition + 1);
        // match mime type to extension
        for (int index = 0; index < mimeType.length; index++) {
            if (fileExtension.equalsIgnoreCase( extension[index])) {
                contentType = mimeType[index];
                break;
            }
        }
        return contentType;
    }
 
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String filename = request.getParameter("filename");
        String realname = request.getParameter("realname");
        String path = "C:\\web\\ftp_file\\";
        String fullPath = path + filename;
        File file = new File(fullPath);
        String contentType = getContentType(filename);
        System.out.println(contentType);
        response.setContentType(contentType);
        response.setHeader("Content-Disposition", "attachment; filename=" + realname);
        int length = (int) file.length();
 
        if (length > Integer.MAX_VALUE) {
        }
 
        byte[] bytes = new byte[length];
 
        FileInputStream fin = new FileInputStream(file);
 
        fin.read(bytes);
 
        ServletOutputStream os = response.getOutputStream();
        os.write(bytes);
        os.flush();
    }
 
    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /** 
     * Handles the HTTP <code>GET</code> method.
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }
 
    /** 
     * Handles the HTTP <code>POST</code> method.
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }
 
    /** 
     * Returns a short description of the servlet.
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}