When you compose a new message on iMessage, below the text field where you type your message is a scrolling bar of app icons that can be invoked and do stuff wi

When you compose a new message on iMessage, below the text field where you type your message is a scrolling bar of app icons that can be invoked and do stuff with the message you are creating. Apps can appear here and provide functions. The built-in Photos app is probably the most used one, typically you open it to create or select a picture to attach. I'm not sure of the API or what it allows you to do, but it probably uses that same mechanism.