Remove application/x-tar file extension test as it might not exist in…

… nginx config (#5253)

Use text/css instead

program/include/rcmail_install.php

@@ -527,7 +527,7 @@ public function check_mime_extensions()
         $errors = array();
         $types  = array(
             'application/zip'   => 'zip',
-            'application/x-tar' => 'tar',
+            'text/css'          => 'css',
             'application/pdf'   => 'pdf',
             'image/gif'         => 'gif',
             'image/svg+xml'     => 'svg',