Shows how WP core uses the hook upload_ui_over_quota

hook_upload_ui_over_quota.php

<?php

// ./wp-includes/ms-default-filters.php, line 48

add_action( 'upload_ui_over_quota', 'multisite_over_quota_message' );


// ./wp-admin/includes/media.php, line 2256-2263

/**
 * Displays the out of storage quota message in Multisite.
 *
 * @since 3.5.0
 */
function multisite_over_quota_message() {
    echo '<p>' . sprintf( __( 'Sorry, you have used all of your storage quota of %s MB.' ), get_space_allowed() ) . '</p>';
}