backupwordpress clobbers upload_mimes() filter

When using BUWP 1.6.6 and WP3.3.1, this function in my functions.php file has no effect:

add_filter('upload_mimes', 'custom_upload_mimes'); 

function custom_upload_mimes ( $existing_mimes=array() ) {
  $existing_mimes['wif'] = 'text/plain';
  return $existing_mimes;
}

Since it works when I deactivate BUWP, I can only suppose that BUWP is removing the filter. Why would it need to do that and how can I make it stop?

http://wordpress.org/extend/plugins/backupwordpress/