It’s an issue with Safari, which has been widely reported not working for popular websites such as Gmail, Facebook, and even Discourse.
See the following links:
https://discussions.apple.com/thread/7727011
https://www.reddit.com/r/mac/comments/3ompay/cant_paste_images_into_facebook_chat_on_safari/?st=iv1bn4zh&sh=0772361d
https://meta.discourse.org/t/paste-images-in-composer-not-working-in-safari/24632