Frostbite is part of the new Deep Freeze bundle that comes with the "physical" (it's a download code) retail version of the game and is available for Nintendo Switch, PlayStation 4, and Xbox One. If you are on PC, you can purchase it in the in-game store. You also get a glider, pickaxe, back bling, and 1,000 V-Bucks with your purchase.
If you want to grab it on console, head to this Amazon (affiliate link) link.
The bundle does NOT come with Save the World.
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Attachments are items that come with a cosmetic when a particular one is purchased.