@RequestMapping(value = {"/api/upload"}, method = RequestMethod.POST)
public ResponseEntity<Map<String, Integer>> upload(@Valid FileBucket fileBucket, BindingResult result) throws IOException {
        Session session = sessionFactory.openSession();
        User user = (User) session.load(User.class, getUserId());

        Map<String, Integer> model = new HashMap<String, Integer>();
        if (result.hasErrors()) {
            System.out.println("validation errors");
            System.out.println(result);
            session.close();
            return new ResponseEntity<>(HttpStatus.INTERNAL_SERVER_ERROR);
        } else {
            int documentId = saveDocument(fileBucket, user);
            model.put("documentId", documentId);
            session.close();
            return new ResponseEntity<Map<String, Integer>>(model, HttpStatus.OK);
        }
}

public class FileBucketConversation {

    private MultipartFile file;

    public MultipartFile getFile() {
        return file;
    }

    public void setFile(MultipartFile file) {
        this.file = file;
    }
}

@Bean(name = "multipartResolver")
public StandardServletMultipartResolver resolver() {
    return new StandardServletMultipartResolver();
}

@Bean(name="multipartResolver")
public CommonsMultipartResolver multipartResolver () {
    CommonsMultipartResolver resolver = new CommonsMultipartResolver();
    resolver.setMaxUploadSize(MAX_FILE_SIZE);
    return resolver;
}

@Bean
@Order(0)
public MultipartFilter multipartFile() {
    MultipartFilter multipartFilter = new MultipartFilter();
    multipartFilter.setMultipartResolverBeanName("multipartResolver");
    return multipartFilter;
}

spring.http.multipart.enabled=true
spring.http.multipart.max-file-size=20Mb
spring.http.multipart.max-request-size=20Mb

spring:
  http:
    multipart:
      enabled: false

import org.springframework.web.multipart.MultipartResolver;
import org.springframework.web.multipart.commons.CommonsMultipartResolver;
...

@Bean
public MultipartResolver multipartResolver() {
    return new CommonsMultipartResolver();
}

@PostMapping(value = "api/upload", consumes = "multipart/form-data")
public void enablePurchase(@RequestHeader HttpHeaders headers,
                           FileBucketConversation fileBucketConversation) {
...
}

/* DON'T USE THIS:
let boundary = "ABDCE";
headers.append("Content-type", "multipart/form-data;boundary=" + boundary);
headers.append("enctype", "multipart/form-data;boundary=" + boundary);
headers.append("boundary", boundary);
*/

const formData = new FormData();
const fileMetadata = {filename: fileName, contentType: 'image/png', knownLength: fs.statSync(pathToFile)};
const formData.append('file', fs.createReadStream(pathToFile), fileMetadata);
fetch(url, {method: 'POST', body: formData});