How slack web client parse URL and response the target URL image and text?

If I post a text includes an article URL to Slack chatroom then check Network requests with Chrome Network pane. None of all the responses include parsed image URL and text.

I post a text https://stackoverflow.com/questions/4140324/parse-url-with-javascript on slack chatroom then I checked the responses data from Networks pane, there is no image URL or text. But slack web client knows what URL and text and showed them on the webpage.

How can slack web app knows the 4th request target URL https%3A%2F%2Fcdn.sstatic.net%2FSites%2Fstackoverflow%2Fimg%2Ffavicon.ico%3Fv%3D4f32ecc8f43d without the response from server?

Or there are some responses data I cannot know from the Network pane.