Disable "Remove File From Disk" for in-progress downloads
(to not break download manager!)
(#15)
Also check using OS.File.exists(), if isn't successfully downloaded
See DownloadsViewItem.prototype.verifyTargetExists():
    // We don't need to check if the download is not finished successfully.
    if (!this.dataItem.openable) {
      return;
    }
(#15)