Spring URI PathVariable with file extension not working

Question

I'm having trouble working out why the file extension (.jpg) in the following request is being stripped before calling my controller method:

GET http://blah.com/assets/picture.jpg

I've set the content negotiation to not favour path extensions:

@Configuration
@EnableWebMvc
@ComponentScan(basePackages = { "com.blah" })
public class PlatformWebAppConfig extends WebMvcConfigurerAdapter {    

    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {    
        configurer.favorPathExtension(false);
        configurer.favorParameter(false);
        configurer.useJaf(false);
        configurer.defaultContentType(MediaType.APPLICATION_JSON);
    }
}

my controller is this:

@Controller
public class FileUploadRestApi {
    @RequestMapping(
        value = "/assets/{filename}",
        method = RequestMethod.GET)
    public void downloadFile(HttpServletResponse response,
        @PathVariable("filename") String filename,
        Principal principal) {
            // ERROR: 'filename' has extension stripped !!!!
    }
}

I've also tried adding the following to the PlatformWebAppConfig class above with no luck:

@Override
public void configurePathMatch(PathMatchConfigurer matcher) {
    matcher.setUseSuffixPatternMatch(true);
    matcher.setUseRegisteredSuffixPatternMatch(true);
}

I've tried to debug the issue and I've got as far as seeing ContentNegotiationManagerFactoryBean.afterPropertiesSet is definitely seeing the favorPathExtension setting as false and not setting up a content negotiation strategy (as expected).

Update:

If I change the request mapping to

"/assets/{filename:.+}"

the filename will include the extensions but this does not explain why setting all those configurers did not achieve anything????