@Override
    public String resolveContentType(String resourceName) {
        if (resourceName != null && !resourceName.isEmpty()) {
            String lowerResourceName = resourceName.toLowerCase();
            String fileExtension = StringUtils.substringAfterLast(lowerResourceName, '.');
            return fileExtensionToContentType.getOrDefault(fileExtension, unknownFileContentType);
        }
        return null;
    }
@Test
    void missingResourceName() {
        ContentTypeResolver contentTypeResolver = new DefaultContentTypeResolver();
        assertThat(contentTypeResolver.resolveContentType(null)).isNull();
        assertThat(contentTypeResolver.resolveContentType("")).isNull();
    }