Add allowed file type to WordPress multisite

The allowed file types list on a multisite is saved in a site option. To add a file type to the allowed uploads list, you can use this code:

add_filter('site_option_upload_filetypes', function($file_types) {
  return rtrim($file_types) . ' ico ';
});

In this case, we are adding the “ico” file extension to allow for uploading of favicons to the media library.