Steps to reproduce:
Browser: Chrome 83.0.4103.97
Can confirm.. (the extension should be .excalidraw)
Duplicate of https://github.com/excalidraw/excalidraw/issues/1624 --- I assume we're talking about the new FS API since you're on CH 83. That said, I thought this bug was resolved in a recent PR. I'm getting lost in the FS issues lately :).
Thanks for the response!