<?php
$types = array
(
  'exe' => 'exe|com',
  'jpg' => 'jpg|jpeg|gif|bmp|pcx|png',
  'zip' => 'zip|alz|gz|tar|z|rar|ace|bz|bz2',
  'mov' => 'avi|mpg|mpeg|mqv|asf|wmv|mov',
  'snd' => 'wav|mp3|mid|wma',
);

$ext = strtolower('ASF');
$r = preg_grep("/$ext/",$types);
$key = key($r);

print_r($key); // 'mov'
?>