There are a bunch of posts about this and it seems there is no definitive answer.
I want to be able to change the images used in an attachment at run-time and more importantly I don't want that image stored or hardcoded in the main spine file. An example of use would be welding a different sword. I might have hundreds of different swords that need to be attached and I don't want to have to embed them in the main spine files atlas or even rebuild the main spine file if a new sword is added. I've heard hints that this is possible but never seen an example of it's use.