One small nit. I would replace this code:

if ($dot > -1) {
    $extension = substr($filename, $dot+1);
    $extensions{$extension}++;
} else {
    $extensions{'_without_extension'}++;
}
[download]

by:

$extensions{$dot == -1 ? '' : substr( $filename,$dot )}++;
[download]

for two reasons:

1. it's more compact
2. it allows you to differentiate between filenames without extension (no . found) and with an empty extension (a . at the end).

Liz