Here is the output from the handy `file` BSD utility which gave it away to me (see the comment field).

    JPEG image data, JFIF standard 1.01, resolution (DPI), density 300x300, segment length 16, comment: "<html><body><style>body { visibility: hidden; } .n { visibility: visible; position: absolute; ", baseline, precision 8, 1000x667, frames 3