Problem with avatar.php

Hi

When i upload the avatar.php,i have this message

Fatal error: Cannot redeclare wp_check_filetype() (previously declared in /home/www/***/wp-includes/functions.php:1624) in /home/www/client/www/cdurablogs/wp-content/mu-plugins/avatar.php on line 11

Any idea?
I use the MU 2.6.1 and the 1815151422_avatars-3.1.0.zip