You don't actually need to buy the items to get the files, you can just grab them from the preview in the shop.
For this post specifically you can just copy the video link directly (using a web browser, not the steam client), but other types of items require dev tools/inspect element.
I had some points left over and bought it. The file can easily be extracted from your Steam folder. Would this be considered a paid reward?
Even if it cost real money I'd liken it more to doujins or commercial game official art, but you just buy it with steam points, so I'd assume it would be fine.
