Yeah, I decided it's best to store the ID, then what you do with files on output (especially images) is more flexible. You need to look at something like:
<?php echo wp_get_attachment_image( slt_cf_field_value( 'product-image-1' ) ); ?>
See http://codex.wordpress.org/Function_Reference/wp_get_attachment_image and the other attachment functions listed at the end of that page.
The "attachment" type you're looking at is probably a "box type", i.e. it says that the "box" of fields being defined is to be included on the attachment edit screens (not post or user edit screens). The "box" concept is a bit redundant for attachment and user fields, but the code started life just dealing with post edit screen meta boxes.