When fully open, an optional header and close icon appears. Clicking the close icon should close the attachment. <img width="395" alt="Screen Shot 2021-02-19 at 4 58 04 PM" src="https://user-images.githubusercontent.com/3860311/108566006-b1b49580-72d3-11eb-8ce8-fc8bde0fcbb9.png"> @ampproject/wg-stories