Upload_MultiPart_Using_Webclient.java

@PostMapping(value = "/upload-multiValueMap", consumes = MediaType.MULTIPART_FORM_DATA_VALUE, produces = MediaType.APPLICATION_STREAM_JSON_VALUE)
@ResponseStatus(value = HttpStatus.OK)
public Mono<YourResponseClass> uploadFileMap(@RequestBody Mono<MultiValueMap<String, Part>> filePartMapMono) {
    
    return filePartMapMono.flatMap(this::sendFile);
}

private Mono<YourResponseClass> sendFile (MultiValueMap<String, Part> filePartMap) {
  return webClient.post()
            .uri("YOUR_ENDPOINT")
            .contentType(MediaType.MULTIPART_FORM_DATA)
            .body(BodyInserters.fromMultipartData(filePartMap))
            .retrieve()
            .bodyToMono(YourResponseClass.class);
}