I learned about X-Content-Type-Options on Hasegawa Yosuke's blog. But his article is little misleading. Content-sniffing is long-lived feature. In the old days, Marc Andreessen wrote about his (Mosaic's) first implementation of IMG element: For the time being, inlined images won’t be explicitly content-type’d; down the road, we plan to support that (along with the general adaptation of MIME). Actu