File Browser: FilePath traversal in download-as-zip/tar via Windows-style backslash separators in stored filenames

Summary filebrowser builds the download-as-zip / download-as-tar archive entry names with filepath.ToSlash, which on a Linux host is a no-op for backslashes \ is only a path separator on Windows. A file whose name contains Windows-style traversal ......\evil.txt is accepted by the resource...