This is how #Slack adds link previews to Slack messages.
on 02016-01-11#Slack messages support “attachments” with titles, summary text, and images, and their “link unfurling” feature “automatically creates attachments” from contents of URLs.
on 02016-01-11“When you share a link in a #Slack message, the link preview contains embedded information from #Twitter-Cards, #oEmbed, or #Open-Graph.” Apparently this is how to control how your links preview in Twitter and Fecebutt too. Also works in #RocketChat.
on 02016-01-11documentation of what part of #slack message formatting is done by the sender, what part by the receiver, and what part by the server
on 02015-08-05#slack fucked-up markup documentation
on 02015-08-05#slack chat message format documented (types, subtypes, stars, pins, reactions)
on 02015-08-05