Spring PartFile name lost when building a MultiPart

As requested here is the new issue @poutsma

My server receive a multipart, take the FilePart from the incoming request and rename it in the outgoing request. Something like that:

  public Mono<ResponseEntity<SomeEntityId>> upload(
      @RequestBody Mono<MultiValueMap<String, Part>> parts) {
    return someServiceClient.upload(parts.map(m -> (FilePart) m.get("foo").get(0));    //here I take the foo part 

  }

  public Mono<ResponseEntity<SomeEntityId>> upload(
      Mono<MultiValueMap<String, Part>> partsMono) {
    return multipartFile.flatMap(file -> {
            MultipartBodyBuilder multipartBodyBuilder = new MultipartBodyBuilder();
            multipartBodyBuilder.part("bar", file); //here I rename it to bar
            return webClient.post()
                    .uri("uri")
                    .header(HttpHeaders.CONTENT_TYPE, MediaType.MULTIPART_FORM_DATA.toString())
                    .body(BodyInserters.fromMultipartData(multipartBodyBuilder.build()))
        });
    }

I came across this issue when upgrading my spring version : when I do this, in the outgoing request, the part that I wanted to rename "bar" is still named "foo". It worked in spring 5.2.8 (it renamed the part) but in 5.3.6 the file is not renamed anymore. I think it's related to this commit e537844 and to the issue #26410

Do you have any advice ? Thanks.