Unicode handling breaks Jpeg image detection?

[Chong Yidong]

In Emacs 23, jpeg content autodetection seems to be broken.  I think
this may be an encoding issue.  To see this, rename any jpeg image from
foo.jpg to foo.JPG (to turn off file-name detection), and open it in
Emacs.  Emacs 23 fails to detect the image, while Emacs 22 works.

In a buffer containing the exact same jpeg data, do

  M-: (string-match "\\`\xff\xd8" (buffer-string)) RET

This gives 0 in Emacs 22, and nil in Emacs 23.  This is the check that
makes image-type-header-regexps and image-jpeg-p fail for Emacs 23.

Could someone try to fix this?