Fix attachment import

You are currently using `_wp_attachment_metadata` to read attachment informations from WP posts. This wont work, because the meta key `_wp_attachment_metadata` only exists for images. In case of non-images, this produces an error while importing.

You are using it to get the file location. A better (and working) way is to read the meta key `_wp_attached_file`, which exists for all attachments (and provides everything you need).