Source

File: wp-includes/class-wp-oembed.php

private function _fetch_with_format( $provider_url_with_args, $format ) {
		$provider_url_with_args = add_query_arg( 'format', $format, $provider_url_with_args );

		/** This filter is documented in wp-includes/class-wp-oembed.php */
		$args = apply_filters( 'oembed_remote_get_args', array(), $provider_url_with_args );

		$response = wp_safe_remote_get( $provider_url_with_args, $args );
		if ( 501 == wp_remote_retrieve_response_code( $response ) ) {
			return new WP_Error( 'not-implemented' );
		}
		$body = wp_remote_retrieve_body( $response );
		if ( ! $body ) {
			return false;
		}
		$parse_method = "_parse_$format";
		return $this->$parse_method( $body );
	}