Super easy trick to find a file extension:
$file="some_file.jpg";
$ext=end(explode(".", $file);
Other ways:
Substring:
$ext = substr($file, strrpos($file, '.') + 1);
//or:
$ext= substr(strrchr($file, "."), 1 );
Using path info:
$info = pathinfo($file);
$ext=$info['extenstion'];