public void addFileExtensionMapping(String fileExtension, String contentType) {
        this.fileExtensionToContentType.put(fileExtension, contentType);
    }
@Test
    void addFileExtensionMapping() {
        DefaultContentTypeResolver contentTypeResolver = new DefaultContentTypeResolver();
        contentTypeResolver.addFileExtensionMapping("docx", "application/vnd.openxmlformats-officedocument.wordprocessingml.document");
        assertThat(contentTypeResolver.resolveContentType("test.docx")).isEqualTo("application/vnd.openxmlformats-officedocument.wordprocessingml.document");
        assertThat(contentTypeResolver.resolveContentType("test.doc")).isEqualTo("application/octet-stream");
    }