Make sure the parameter name "file" is correct?
Try this, this is how I use it, no problem.

List<String> imgUrls=new ArrayList<>();
MultipartHttpServletRequest multipartRequest =(MultipartHttpServletRequest) request;
List<MultipartFile> files=multipartRequest.getFiles("file");

Plus front-end

<form action="/upload" method="post" enctype="multipart/form-data">
<input type="file" name="file" multiple>
<input type="submit" ">
</form>

var formData = new FormData();
formData.append('file', $('#****')[0].files[0]);

I just encountered this problem today, and I solved it after research.

This is because the configuration of the ServletRegistrationBean injected by yourself is different from the ServletRegistrationBean injected by spring boot by default. The ServletRegistrationBean you injected yourself is not configured to handle upload requests by default.

Please refer to the following configuration:

    //自动注入spring boot默认的上传配置
    @Autowired
    private MultipartConfigElement multipartConfigElement;
    @Bean
    public ServletRegistrationBean apiV1ServletBean(WebApplicationContext wac) {
        DispatcherServlet servlet = new DispatcherServlet(wac);
        ServletRegistrationBean bean = new ServletRegistrationBean(servlet, "/api/*");
        //注入上传配置到自己注册的ServletRegistrationBean
        bean.setMultipartConfig(multipartConfigElement);
        bean.setName("ApiServlet");
        return bean;
    }