Currently you can only upload attachements. For teams that work in tools requiring online usage (such as Adobe XD, where design review can be performed via a link) it'd be good to be able to add that key asset link as an attachment rather than having to include as a comment which could be lost.
Thanks for sharing this idea! This is possible today with custom fields. You can create a URL/Email field that would serve as a clickable link to your Adobe XD files.