On a card, there is the option for attachments. Clicking on attachments one of the options is to Attach a link.
I would like to attach a link to a file in iCloud rather than attach a file. The reason is if I make changes to a file in iCloud or any other linked file then the changes will be reflected when it is opened.
In the attach file option it appears to attach a copy of the original file and changes that are made are only reflected in the copy.