See code:
var file1 = \"50.xsl\";
var file2 = \"30.doc\";
getFileExtension(file1); //returns xsl
getFileExtension(file2); //returns doc
function getFileExt
Newer Edit: Lots of things have changed since this question was initially posted - there's a lot of really good information in wallacer's revised answer as well as VisioN's excellent breakdown
Edit: Just because this is the accepted answer; wallacer's answer is indeed much better:
return filename.split('.').pop();
My old answer:
return /[^.]+$/.exec(filename);
Should do it.
Edit: In response to PhiLho's comment, use something like:
return (/[.]/.exec(filename)) ? /[^.]+$/.exec(filename) : undefined;