USB sticks are prone to failure. Btw, it is alright to rename it to any format.
Use multiple ones, 1 usb costs about 3-4$ here. You can easily buy 10 of them and then put them in a faraday box to shield against EMP charges.
I dont know about renaming, certain formats have different encoding and it might fuck up the data. Like if you rename a .mp3 to .txt, and then back to .mp3 it wont work anymore.
On USB or Paper wallet, or any back up have more then 1. If you just have one and it has something wrong... single point of failure so very bad.
Multiple backup's and you can also do saving on USB and paper wallet. So you can combine multiple. And just make sure to store in safe place, preferably 2 safe places. As you don't want 1 point of failure.