Source

File: wp-includes/widgets/class-wp-widget-media-video.php

public function get_instance_schema() {

		$schema = array(
			'preload' => array(
				'type'                  => 'string',
				'enum'                  => array( 'none', 'auto', 'metadata' ),
				'default'               => 'metadata',
				'description'           => __( 'Preload' ),
				'should_preview_update' => false,
			),
			'loop'    => array(
				'type'                  => 'boolean',
				'default'               => false,
				'description'           => __( 'Loop' ),
				'should_preview_update' => false,
			),
			'content' => array(
				'type'                  => 'string',
				'default'               => '',
				'sanitize_callback'     => 'wp_kses_post',
				'description'           => __( 'Tracks (subtitles, captions, descriptions, chapters, or metadata)' ),
				'should_preview_update' => false,
			),
		);

		foreach ( wp_get_video_extensions() as $video_extension ) {
			$schema[ $video_extension ] = array(
				'type'        => 'string',
				'default'     => '',
				'format'      => 'uri',
				/* translators: %s: Video extension. */
				'description' => sprintf( __( 'URL to the %s video source file' ), $video_extension ),
			);
		}

		return array_merge( $schema, parent::get_instance_schema() );
	}