Swagger 2 not displaying model for Maps

Question

I have swagger2 with springfox configured on a Spring MVC app exposing rest services. One of the exposed services has a map as Requestbody parameter:

@RequestMapping(value = "/uploadFiles", method = RequestMethod.POST, produces= "application/json", headers = "Accept=application/json")
    @ResponseBody
    public ResponseEntity<String> uploadFilesToGED(@RequestParam(value = "userId") String userId, @RequestBody Map<String,DocumentMetaData> documents  ) {
        GEDService.pushDocument(userId, documents);
        return new ResponseEntity<>(HttpStatus.ACCEPTED);
    }

The problem is that swagger-ui doesn't show me a blank model for the map attribute but only a pair of empty brackets {} . I would have expected the model to show me a default map representation with a String and the model of the object DocumentMetaData (which contains himself other Maps and Lists...):

So after googling a while I found some posts talking about ObjectMapper configuration. Mine is very simple:

public class RestAPIConfig extends WebMvcConfigurerAdapter {
...
        /**
     * configureMessageConverters
     */
    @Override
    public void configureMessageConverters(
            List<HttpMessageConverter<?>> converters) {
        converters.add(jackson2Converter());
        converters.add(new ByteArrayHttpMessageConverter());
        converters.add(new ResourceHttpMessageConverter());
        converters.add(new SourceHttpMessageConverter<Source>());
        converters.add(new AllEncompassingFormHttpMessageConverter());
        converters.add(new ObjectHttpMessageConverter());
           }

        /**
     * MappingJackson2HttpMessageConverter
     * 
     * @return Converter from java to json
     */
    @Bean
    public MappingJackson2HttpMessageConverter jackson2Converter() {
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        converter.setObjectMapper(objectMapper());
        return converter;
    }

    /**
     * Object mapper
     * 
     * @return objectMapper
     */
    @Bean
    @Primary
    public ObjectMapper objectMapper() {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.enable(SerializationFeature.INDENT_OUTPUT);
        return objectMapper;
    }
...
}

Why does I see empty brackets instead of a blank representation of the map?

Thx