Source

File: wp-includes/class-wp-xmlrpc-server.php

public function attach_uploads( $post_ID, $post_content ) {
		global $wpdb;

		// Find any unattached files.
		$attachments = $wpdb->get_results( "SELECT ID, guid FROM {$wpdb->posts} WHERE post_parent = '0' AND post_type = 'attachment'" );
		if ( is_array( $attachments ) ) {
			foreach ( $attachments as $file ) {
				if ( ! empty( $file->guid ) && strpos( $post_content, $file->guid ) !== false ) {
					$wpdb->update( $wpdb->posts, array( 'post_parent' => $post_ID ), array( 'ID' => $file->ID ) );
				}
			}
		}
	}