imagemagick format detection code

[Lars Magne Ingebrigtsen <larsi@gnus.org>]

duckduckgo outputs images as gzipped .ico files, and eww can now almost
display them.  But!  If I create the image from the data in the buffer,
I just get a square which imagemagick uses to tell us "nuh-huh; don't
understand": 

(with-temp-buffer
  (set-buffer-multibyte nil)
  (insert-file-contents-literally "/tmp/b.ico")
  (let ((image (create-image (buffer-string) 'imagemagick t)))
    (pop-to-buffer "*image*")
    (insert-image image "*")))

However, if I create the image with

(create-image "/tmp/b.ico" 'imagemagick)

then it works fine.

Does anybody have any, er, insights to offer here?

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