Optional message properties that can be sent to things like prompts or session.say().
Hint for how clients should layout multiple attachments. The default value is 'list'.
For incoming messages contains attachments like images sent from the user. For outgoing messages contains objects like cards or images to send to the user.
Structured objects passed to the bot or user.
Hint for clients letting them know if the bot is expecting further input or not. The built-in prompts will automatically populate this value for outgoing messages.
Format of text fields. The default value is 'markdown'.