I have a website that allows users to enter data and download the files as JSON. For example, they could make a custom card and download the card into a file with the JSON format. They can even have a list of cards and download the whole deck into a file with the JSON format.

My question is, what file extension should they have? Is it best practice for the files to all have the .JSON extension to say that they’re json formatted? Or should I make new extensions .CARD for card files and .DECK for deck files?