Spring webflux WebClient将文件发布到客户端

我经常会发现一个常见的错误,但我没有尝试过解决它。

@GetMapping("process")
public Flux<String> process() throws MalformedURLException {
    final UrlResource resource = new UrlResource("file:/tmp/document.pdf");

    MultiValueMap<String, UrlResource> data = new LinkedMultiValueMap<>();
    data.add("file", resource);

    return webClient.post()
            .uri(LAMBDA_ENDPOINT)
            .contentType(MediaType.MULTIPART_FORM_DATA)
            .body(BodyInserters.fromMultipartData(data))
            .exchange()
            .flatMap(response -> response.bodyToMono(String.class))
            .flux();
}

我正在AWS Lambda中使用它,具有以下端点:

    @PostMapping(path = "/input", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
    public ResponseEntity<List<?>> input(@RequestParam("file") MultipartFile file) throws IOException {
        final ByteBuffer byteBuffer = ByteBuffer.wrap(file.getBytes());

        //[..]

        return new ResponseEntity<>(result, HttpStatus.OK);
    }

但我一直在想:

{  
   "timestamp":1549395273838,
   "status":400,
   "error":"Bad Request",
   "message":"Required request part 'file' is not present",
   "path":"/detect-face"
}

从lambda回来;

我只是错误地设置了文件的发送,还是需要在API网关上配置一些东西以允许请求参数进入?