chore(cache): only cache header info for file type of image

[XuNeo]

Description of the feature or fix

For variable type of image, the header cache won't be that useful considering that PNG, JPG are easy to get info.
Furthermore, if the image has changed, which is usual for variable type of image in RAM, user must manually invalidate the cache.

So it's better to only add header to cache for file type of image.

cc @W-Mai

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