By default, WordPress only enables the following file types (extensions really) to be uploaded:

jpg jpeg png gif mp3 mov avi wmv midi mid pdf flv

If you want to enable tar or for example doc file, you need to edit a database entry. Go to wp_sitemeta table and search for an entry identified by upload_filetypes name in meta_key column. Enter the extensions separated by space and save the entry.

If you use object cache, clean it after to have the effect.