Allow Package/Index file with an empty extension

When downloading Index files during an "update" operation there are
often "Index" files without an extension at all. So additionally to
"Index" extensions like: .gz, .lzma, .bz2 also try to download a
file/package without any filename extension.
This was by intention. I used the Python keyword None because I check this in line 828 of the patch: Dependinn on whether the format (file extension) is None or something else I either simply use the passed file basename or append dot + file extension.