WP_REST_Attachments_Controller::get_item_schema()

	public function get_item_schema() {

		$schema = parent::get_item_schema();

		$schema['properties']['alt_text'] = array(
			'description'     => __( 'Alternative text to display when attachment is not displayed.' ),
			'type'            => 'string',
			'context'         => array( 'view', 'edit', 'embed' ),
			'arg_options'     => array(
				'sanitize_callback' => 'sanitize_text_field',
			),
		);

		$schema['properties']['caption'] = array(
			'description' => __( 'The attachment caption.' ),
			'type'        => 'object',
			'context'     => array( 'view', 'edit', 'embed' ),
			'arg_options' => array(
				'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
				'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
			),
			'properties'  => array(
				'raw' => array(
					'description' => __( 'Caption for the attachment, as it exists in the database.' ),
					'type'        => 'string',
					'context'     => array( 'edit' ),
				),
				'rendered' => array(
					'description' => __( 'HTML caption for the attachment, transformed for display.' ),
					'type'        => 'string',
					'context'     => array( 'view', 'edit', 'embed' ),
					'readonly'    => true,
				),
			),
		);

		$schema['properties']['description'] = array(
			'description' => __( 'The attachment description.' ),
			'type'        => 'object',
			'context'     => array( 'view', 'edit' ),
			'arg_options' => array(
				'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
				'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
			),
			'properties'  => array(
				'raw' => array(
					'description' => __( 'Description for the object, as it exists in the database.' ),
					'type'        => 'string',
					'context'     => array( 'edit' ),
				),
				'rendered' => array(
					'description' => __( 'HTML description for the object, transformed for display.' ),
					'type'        => 'string',
					'context'     => array( 'view', 'edit' ),
					'readonly'    => true,
				),
			),
		);

		$schema['properties']['media_type'] = array(
			'description'     => __( 'Attachment type.' ),
			'type'            => 'string',
			'enum'            => array( 'image', 'file' ),
			'context'         => array( 'view', 'edit', 'embed' ),
			'readonly'        => true,
		);

		$schema['properties']['mime_type'] = array(
			'description'     => __( 'The attachment MIME type.' ),
			'type'            => 'string',
			'context'         => array( 'view', 'edit', 'embed' ),
			'readonly'        => true,
		);

		$schema['properties']['media_details'] = array(
			'description'     => __( 'Details about the media file, specific to its type.' ),
			'type'            => 'object',
			'context'         => array( 'view', 'edit', 'embed' ),
			'readonly'        => true,
		);

		$schema['properties']['post'] = array(
			'description'     => __( 'The ID for the associated post of the attachment.' ),
			'type'            => 'integer',
			'context'         => array( 'view', 'edit' ),
		);

		$schema['properties']['source_url'] = array(
			'description'     => __( 'URL to the original attachment file.' ),
			'type'            => 'string',
			'format'          => 'uri',
			'context'         => array( 'view', 'edit', 'embed' ),
			'readonly'        => true,
		);

		unset( $schema['properties']['password'] );

		return $schema;
	}