there's a truly dubious test case in the File Extension test. file called .htaccess is not filename "" and extension "htaccess" with a "." separating the two. .htaccess is a unix style dotfile. its whole name is ".htaccess" and it has no file extension.
I have to admit to cheating and putting 'if (i === '.htaccess') return 'htaccess';' to save time :)
return i.indexOf('.') > -1 ? i.substring(i.indexOf('.')+1) : false;
Seemed to pass all test cases