Testing MockRestServiceServer spring-test with multipart request

Recently I've started to use Spring's MockRestServiceServer to verify my RestTemplate based requests in tests.

When its used for simple get/post request - all good, however, I couldn't figure out how to use it with POST multipart request:

For example, my working code that I would like to test looks like this:

public ResponseEntity<String> doSomething(String someParam, MultipartFile 
   file, HttpHeaders headers) { //I add headers from request 

   MultiValueMap<String, Object> map = new LinkedMultiValueMap<>();
   map.add("file", new ByteArrayResource(file.getBytes()) {
            @Override
            public String getFilename() {
                return file.getOriginalFilename();
            }
        });
        map.add("someParam", someParam);
        HttpEntity<MultiValueMap<String, Object>> requestEntity = new 
             HttpEntity<>(map, headers);
        return this.restTemplate.exchange(
                  getDestinationURI(), 
                  HttpMethod.POST, 
                  requestEntity, 
                  String.class);
}

So my question is How I can specify my expectations with org.springframework.test.web.client.MockRestServiceServer? Please notice, that I don't want to just mock the "exchange" method with mockito or something, but prefer to use MockRestServiceServer

I'm using spring-test-4.3.8.RELEASE version

A code snippet would be really appreciated :)

Thanks a lot in advance

Update: As per James's request I'm adding non-working test snippet (Spock test):

MockRestServiceServer server = MockRestServiceServer.bindTo(restTemplate).build()
        server.expect(once(), requestTo(getURI()))
             .andExpect(method(HttpMethod.POST))
             .andExpect(header(HttpHeaders.CONTENT_TYPE, startsWith("multipart/form-data;boundary=")))

             .andExpect(content().formData(["someParam" : "SampleSomeParamValue", "file" : ???????] as MultiValueMap))
             .andRespond(withSuccess("sample response", MediaType.APPLICATION_JSON))

        multipartFile.getBytes() >> "samplefile".getBytes()
        multipartFile.getOriginalFilename() >> "sample.txt"

I get exception while asserting the request content. The form data is different, because an actual form data is created internally with Content-Disposition, Content-Type, Content-Length per parameter and I don't know how to specify these expected values