Hello @Leo_Lhuile!
I am sorry you are having these issues with the zip being corrupted, very odd bug. It seems like you are using the right functions to grab the data and convert it to a processable format.
I can try to repro this and see if I am getting the same error. If so I can check with our engineering team to see why the utils.downloadFile() isn't working and see if we have a workaround or another method for using such with atob().
On a side note, I found another user with a similar post, where another community member had a recommendation for using JS libraries tmp with fs! Let me know if that works for you 