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http://www.20min.ch/diashow/42267/pravda2-1332eceabdde503907ab4e52dbd37870.jpeg is a JPEG sent with 'Content-Type: text/plain' and 'Content-Encoding: gzip' headers. The spec should clarify that sniffing should be done after the resource has been decoded.
This will fall out of Fetch/HTML integration.