How to use the @hint/utils-fs.fileExtension function in @hint/utils-fs

const validate = ({ element, resource }: ElementFound) => {
            if (normalizeString(element.getAttribute('rel')) === 'manifest') {
                const href = element.resolveUrl(element.getAttribute('href') || /* istanbul ignore next */ '');
                const fileExtension: string = getFileExtension(normalizeString(href) || /* istanbul ignore next */ '');

                if (fileExtension !== standardManifestFileExtension) {
                    let message: string;
                    let severity: Severity;

                    if (fileExtension) {
                        message = getMessage('shouldHaveFileExtensionNot', context.language, [standardManifestFileExtension, fileExtension]);

                        severity = fileExtension === 'json' ?
                            Severity.hint :
                            Severity.warning;
                    } else {
                        message = getMessage('shouldHaveFileExtension', context.language, standardManifestFileExtension);
                        severity = Severity.warning;
                    }

const determineMediaTypeBasedOnFileExtension = (resource: string, originalMediaType: string | null = null): string | null => {
    const fileExtension = getFileExtension(resource);

    if (!fileExtension) {
        return null;
    }

    /*
     * The following list is order based on the expected encounter
     * rate and different statistics (e.g. for images, except `ico`,
     * http://httparchive.org/interesting.php#imageformats)
     */

    /*
     * The reasons for hard-coding some of the values here are:
     *
     *  * `mime-db` is quite big, so querying it is expensive.
     *  * `mime-db` sometimes has multiple media types for