Custom fields don't appear on post-type 'attachments'.

Using CustomPress I new created a Custom Field content type for post-type 'attachment'.
The field does not show up on the media page for me to use.
I tried a text field, a radio button, a select box and it didn't seem to matter.
The same custom field does work if I assign it to post-type 'post', so I'm assuming it is something special about the 'attachment' post type?