It looks like this is only in firefox, so we shouldn't use it this way - we would always get `false` here for other browsers. https://developer.mozilla.org/en-US/docs/Web/API/ProgressEvent.lengthComputable

Instead, can do `if (typeof e.lengthComputable === 'undefined' || e.lengthComputable)` so it works if not defined at all, or if defined and true