Return t if the current buffer contains an auto-detectable image.
This function is intended to be used from `magic-fallback-mode-alist'.
The buffer is considered to contain an auto-detectable image if
its beginning matches an image type in `image-type-header-regexps',
and that image type is present in `image-type-auto-detectable' with a
non-nil value. If that value is non-nil, but not t, then the image type must be available.