Source

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

public function add_enclosure_if_new( $post_ID, $enclosure ) {
		if ( is_array( $enclosure ) && isset( $enclosure['url'] ) && isset( $enclosure['length'] ) && isset( $enclosure['type'] ) ) {
			$encstring  = $enclosure['url'] . "\n" . $enclosure['length'] . "\n" . $enclosure['type'] . "\n";
			$found      = false;
			$enclosures = get_post_meta( $post_ID, 'enclosure' );
			if ( $enclosures ) {
				foreach ( $enclosures as $enc ) {
					// This method used to omit the trailing new line. #23219
					if ( rtrim( $enc, "\n" ) == rtrim( $encstring, "\n" ) ) {
						$found = true;
						break;
					}
				}
			}
			if ( ! $found ) {
				add_post_meta( $post_ID, 'enclosure', $encstring );
			}
		}
	}