@RestController
public class UploadController {

  @PostMapping(value = "/uploadFile", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
  public String handleFileUpload(@RequestPart(value = "file") MultipartFile file) {
    return file.getName();
  }
  
}

<dependency>
  <groupId>io.github.openfeign.form</groupId>
  <artifactId>feign-form</artifactId>
  <version>3.0.3</version>
</dependency>
<dependency>
  <groupId>io.github.openfeign.form</groupId>
  <artifactId>feign-form-spring</artifactId>
  <version>3.0.3</version>
</dependency>
<dependency>
  <groupId>commons-fileupload</groupId>
  <artifactId>commons-fileupload</artifactId>
</dependency>

@FeignClient(value = "upload-server", configuration = TestServiceClient.MultipartSupportConfig.class)
public interface UploadService { 
  @PostMapping(value = "/uploadFile", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
  String handleFileUpload(@RequestPart(value = "file") MultipartFile file);
 
  @Configuration
  class MultipartSupportConfig {
    @Bean
    public Encoder feignFormEncoder() {
      return new SpringFormEncoder();
    }
  } 
}

@Test
@SneakyThrows
public void testHandleFileUpload() { 
  File file = new File("files/aaa.txt");
  DiskFileItem fileItem = (DiskFileItem) new DiskFileItemFactory().createItem("file",
      MediaType.TEXT_PLAIN_VALUE, true, file.getName()); 
  try (InputStream input = new FileInputStream(file); OutputStream os = fileItem.getOutputStream()) {
    IOUtils.copy(input, os);
  } catch (Exception e) {
    throw new IllegalArgumentException("Invalid file: " + e, e);
  } 
  MultipartFile multi = new CommonsMultipartFile(fileItem); 
  log.info(testServiceClient.handleFileUpload(multi));
}