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'];