JSP commons-fileupload WAF Bypass

@Override
 protected void doPost(HttpServletRequest request, HttpServletResponse response)
 throws ServletException, IOException {
 
 response.setContentType("text/html; charset=UTF-8");
 request.setCharacterEncoding(CHARSET);
 PrintWriter out = response.getWriter();
 
 File attachesDir = new File(ATTACHES_DIR);
 
 DiskFileItemFactory fileItemFactory = new DiskFileItemFactory();
 fileItemFactory.setRepository(attachesDir);
 fileItemFactory.setSizeThreshold(LIMIT_SIZE_BYTES);
 ServletFileUpload fileUpload = new ServletFileUpload(fileItemFactory);
 
 try {
 List<FileItem> items = fileUpload.parseRequest(request);
 for (FileItem item : items) {
 if (item.isFormField()) {
 System.out.printf("파라미터 명 : %s, 파라미터 값 : %s \n", item.getFieldName(), item.getString(CHARSET));
 } else {
 System.out.printf("파라미터 명 : %s, 파일 명 : %s, 파일 크기 : %s bytes \n", item.getFieldName(),
 item.getName(), item.getSize());
 if (item.getSize() > 0) {
 Object fileName = new File(item.getName()).getName();
 fileName = this.makeFn((String)fileName) + "." + this.getExt((String)fileName); // makeFn -> make random filename
 String filePath = uploadPath + File.separator + (String)fileName;
 File storeFile = new File(filePath);
 item.write(storeFile);
 }
 }
 }
 out.println("<h1>파일 업로드 완료</h1>");
 } catch (Exception e) {
 // 파일 업로드 처리 중 오류가 발생하는 경우
 e.printStackTrace();
 out.println("<h1>파일 업로드 중 오류가 발생하였습니다.</h1>");
 }
 }
 
//출처: https://dololak.tistory.com/720 [코끼리를 냉장고에 넣는 방법]

private String getExt(String fileName) {
 if (fileName.lastIndexOf(".") == -1)
 return "noext"; 
 fileName = fileName.substring(fileName.lastIndexOf(".") + 1).trim();
 if ("".equals(fileName))
 return "noext"; 
 return fileName.substring(fileName.lastIndexOf(".") + 1);
 }

4. Encodings
 
 Initially, the legal values for "encoding" are "Q" and "B". These
 encodings are described below. The "Q" encoding is recommended for
 use when most of the characters to be encoded are in the ASCII
 character set; otherwise, the "B" encoding should be used.
 
 ...중략...
 
 4.1. The "B" encoding
 
 The "B" encoding is identical to the "BASE64" encoding defined by RFC