private void download(HttpServletRequest request, HttpServletResponse response, String fileName){
    String fullPath = this.filePath + fileName;
    File downloadFile = new File(fullPath);
    if (!downloadFile.exists()){
        throw new CustomHttpException(10006);
    }

    ServletContext context = request.getServletContext();
    String mimeType = context.getMimeType(fullPath);
    if (mimeType == null) {
        mimeType = "application/octet-stream";
    }
    try {
        response.setContentType(mimeType);
        response.setContentLength((int) downloadFile.length());
        response.setCharacterEncoding("utf-8");
        String headerKey = "Content-Disposition";
        String headerValue = String.format(
                "attachment; filename=\"%s\"",
                URLEncoder.encode(downloadFile.getName(), "UTF-8")
        );
        response.setHeader(headerKey, headerValue);
        InputStream myStream = new FileInputStream(fullPath);
        IOUtils.copy(myStream, response.getOutputStream());
        response.flushBuffer();
    } catch (IOException ue){
        throw new CustomHttpException(10006);
    }
}