Source

File: wp-admin/includes/class-wp-media-list-table.php

public function current_action() {
		if ( isset( $_REQUEST['found_post_id'] ) && isset( $_REQUEST['media'] ) ) {
			return 'attach';
		}

		if ( isset( $_REQUEST['parent_post_id'] ) && isset( $_REQUEST['media'] ) ) {
			return 'detach';
		}

		if ( isset( $_REQUEST['delete_all'] ) || isset( $_REQUEST['delete_all2'] ) ) {
			return 'delete_all';
		}

		return parent::current_action();
	}