MaxLich

...
 @Bean
    public WebMvcConfigurer webMvcConfigurer() {
        return new WebMvcConfigurer() {

            @Override
            public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
                converters.add(stringConverter());
                converters.add(mappingJackson2HttpMessageConverter());
                converters.add(excelConverter());
            }

...
}

...

    @Bean
    public ResourceHttpMessageConverter excelConverter() {
        final ResourceHttpMessageConverter converter = new ResourceHttpMessageConverter();
        converter.setSupportedMediaTypes(
                List.of(new MediaType("application", "vnd.ms-excel"))
        );
        return converter;
    }

....
      return ResponseEntity
                .ok()
                .headers(headers)
                .contentLength(fileCont.length)
                .contentType(new MediaType("application", "vnd.ms-excel"))
                .body(new InputStreamResource(in));
      }
}

jaxbContext = JAXBContext.newInstance(PACKAGE_WITH_JAXB_OBJECTS + ":" + PACKAGE_WITH_JAXB_OBJECTS_2);

@javax.xml.bind.annotation.XmlSchema(namespace = "http://firma.ru/schema/v6_1/", elementFormDefault = javax.xml.bind.annotation.XmlNsForm.QUALIFIED)
package ru.my-firm.app.my-project.jaxbobjects;

public class WebAppInitializer implements WebApplicationInitializer {
    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        context.register(SpringConfig.class, WebConfig.class);
        context.setServletContext(servletContext);

        ServletRegistration.Dynamic dispatcher = servletContext.addServlet("dispatcher", new DispatcherServlet(context));
        dispatcher.setLoadOnStartup(1);
        dispatcher.addMapping("/");

        CharacterEncodingFilter characterEncodingFilter = new CharacterEncodingFilter();
        characterEncodingFilter.setEncoding("UTF-8");
        characterEncodingFilter.setForceEncoding(true);

        FilterRegistration.Dynamic filterRegistration = servletContext
                .addFilter("characterEncodingFilter", characterEncodingFilter);
        filterRegistration.addMappingForUrlPatterns(null, false, "/*");
    }
}

CharacterEncodingFilter characterEncodingFilter = new CharacterEncodingFilter();
        characterEncodingFilter.setEncoding("UTF-8");
        characterEncodingFilter.setForceEncoding(true);

        FilterRegistration.Dynamic filterRegistration = servletContext
                .addFilter("characterEncodingFilter", characterEncodingFilter);
        filterRegistration.addMappingForUrlPatterns(null, false, "/*");