Enumerating filenames for subscribestar

[Doofy420]

I'm trying to preserve the image post order for galleries on subscribestar, but I can't figure out how to do it.

Ideally, I'd use {post_id}_{num}.{extension}, but {num} returns none in this case, and the default format {post_id}_{id} doesn't work for my intended purpose.

Any solutions?