Clean up KTX/CRN support

There are at least 3 places where we check if a file is KTX, CRN, or a standard web image. This could probably be unified.

We are also missing tests in these places, so once we unify them we could also add a test for it. For example, ImagereyProvider.loadImage does not test KTX/CRN loading.

Start by locating the three file-type checks mentioned in the issue and read ImagereyProvider.loadImage, which is specifically noted as lacking KTX/CRN loading tests. Unify the checks and add coverage for the supported image types, including KTX and CRN, then run the affected test suite to confirm the behavior.

Written by the indexing model from the issue text.