@RestController
@RequestMapping("file-upload")
public class MyRESTController {

  @Autowired
  private AService aService;

  @RequestMapping(method = RequestMethod.POST, consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
  @ResponseStatus(HttpStatus.CREATED)
  public void fileUpload(
      @RequestParam(value = "file", required = true) final MultipartFile file,
      @RequestParam(value = "something", required = true) final String something) {
   aService.doSomethingOnDBWith(file, value);
  }
}

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = MyApplication.class)
@WebAppConfiguration
public class ControllerTest{

    MockMvc mockMvc;

    @Mock
    AService aService;

    @InjectMocks
    MyRESTController controller;

  @Before public void setUp(){
    MockitoAnnotations.initMocks(this);
    this.mockMvc = MockMvcBuilders.standaloneSetup(controller).build();
  }

  @Test
  public void testFileUpload() throws Exception{
        final File file = getFileFromResource(fileName);
        //File is correctly loaded
        final MockMultipartFile multipartFile = new MockMultipartFile("aMultiPartFile.txt", new FileInputStream(file));

        doNothing().when(aService).doSomethingOnDBWith(any(MultipartFile.class), any(String.class));

        mockMvc.perform(
                post("/file-upload")
                        .requestAttr("file", multipartFile.getBytes())
                        .requestAttr("something", ":(")
                        .contentType(MediaType.MULTIPART_FORM_DATA_VALUE))
                .andExpect(status().isCreated());
    }
}

java.lang.IllegalArgumentException: Expected MultipartHttpServletRequest: is a MultipartResolver configured?