The attachment import doesn't work in SMF 2.0 RC2.
I had to change this line in /impex/systems/smf2/010.php:
Canal Rss
Canal Atom$raw_filename = $import_id . '_' . str_replace('.', '_', $data['filename']) . md5($data['filename']);
$raw_filename = $import_id . '_' . $data['file_hash'];