Like in XFS 1.8? I've found in 2.0 if anonymous upload is disabled then it redirects to the login page. If you just want to display an error message instead then do this, in index.cgi put a number sign (#) at the start of lines 431, 432 and 433 like so:

   #$ses->redirect("?op=login") if !$c->{enabled_anon} && !$ses->getUser;
   #$ses->redirect("?op=login") if !$c->{upload_on} && $utype eq 'anon';   
   #$ses->message("Uploads are disabled for your user type","Upload error") if !$c->{upload_on};


Underneath put:

   $ses->message("To start upload. You must be a registered user. Please <a href=\"$c->{site_url}/?op=registration\">register on the site.</a>") if !$c->{upload_on_anon} && $utype eq 'anon';