Thanks for letting me know. The example shown is incorrect as you can't reply to it. Because to reply you need to have a channel id in event object and slack do

Thanks for letting me know. The example shown is incorrect as you can't reply to it. Because to reply you need to have a channel id in event object and slack doesn't send a channel id for file_shared event (https://api.slack.com/events/file_shared https://api.slack.com/events/file_shared). I will modify the example now. Thanks very much.