Use LoadFile only if needed

[RaananW]

Forum discussion here - https://forum.babylonjs.com/t/filetools-why-loadimage-loads-the-image-as-blob/44324

The idea is that if there are no WebRequest modifiers there is no need to use filetools.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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