We will most likely have a problem here because we've never needed to support writing files to multiple platforms. Our tests all use `/` as the destination separator because they're assuming the target is Android. By switching it here to `os.path.sep` this will fail on Windows. What we really need to do is only use `os.path.sep` when we're writing to local file. I think you'll see all of these issues yourself if you run the tests on Windows, rather than having them arise in a code review.