testing needed: only use image loaders which could potentially read file #17368
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Rather than blindly applying image loaders until one succeeds, use magic numbers at the start of the image file to determine which loader(s) to use, and apply only those. Also detect a variety of magic numbers for common non-image formats so that we don't even attempt to load them and can give an appropriately specific error to the user.
This PR will need thorough testing because it changes such a core operation.
It will also have merge conflicts with #17367, which I will fix if both are accepted.