There is no preview if the response content-type header is formatted with quotes around the charset: text/html; charset=”utf-8” instead of the usual text/html; charset=utf-8 .

Reproducing

1. Create a new HTTP GET request.

2. Preferrably set the User-Agent string to be a real browser’s.

3. Do a request against https://www.facebook.com/marketplace/

4. Observe the message: