Oh, my mistake, I meant sv_allowupload and sv_allowdownload. Anyway, it's not relevant.
What I want has nothing to do with these settings. I just intend to block the "Error: server failed to transmit file 'customization" message from showing in the players' consoles.
To be more precise: this error can be avoided by setting sv_allowupload and sv_send_logos to 1, but I can't do that from some reason, so this message keep showing up in the players' consoles.