function normalizeExtension (extension) {

	if (typeof extension !== 'string') {
		throw new Error('`extension` argument must be a string');
	}

	switch (extension) {
		case 'mp4':
		case 'm4v':
			return 'mp4';
		case 'webm':
		case 'webma':
		case 'webmv':
			return 'webm';
		case 'ogg':
		case 'oga':
		case 'ogv':
			return 'ogg';
		default:
			return extension;
	}
}
Get standard extension of a media file

@param {String} extension
@return {String}