JavaScript function to get the file extension from the specified file name. It will return an empty string if you specify an invalid path.

/**
 * Returns the extension of the passed file name. It will return an empty string
 * if you pass an invalid path.
 *
 * @param {String}    path    The fileName path like '/path/to/file.mp4'
 * @returns {String}          The extension in lower case or an empty string if
 *     no extension could be found.
 * @method fileExtension
 */
function fileExtension(path) {
    if (typeof path === 'string') {
        var splitPathRe = /^(\/?)([\s\S]*?)((?:\.{1,2}|[^\/]+?)(\.([^\.\/\?]+)))(?:[\/]*|[\?].*)$/i;
        var pathParts = splitPathRe.exec(path);
        if (pathParts) {
            return pathParts.pop().toLowerCase();
        }
    }
    return '';
}