发送多个文件作为请求的一部分

Question

下面的代码将调用服务以发送带有1个附件的邮件，我想添加更多附件。

private void sendMailWithAttachemnts(String url, Map<String, Object> requestPayload) {
HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.MULTIPART_FORM_DATA);
    FileSystemResource file = new FileSystemResource(new File(invoiceServiceProperties.getInvoiceDir()+"\\"+requestPayload.get("MerchantName")+"_invoice.pdf"));
    MultiValueMap<String, Object> body = new LinkedMultiValueMap<>();
    **body.add("files", file);** // i want to add multiple files here
    body.add("to",requestPayload.get("mercahntEmailId"));
    body.add("emailSubjectLine","Welcome Aboard");
    body.add("emailBody","Welcome aboard."); 
    HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(body, headers);
    RestTemplate restTemplate = new RestTemplate();
    ResponseEntity<String> response = restTemplate.exchange(url,HttpMethod.POST, requestEntity, String.class);
    logger.info("response : {}", response);    
    }