Change image extension priority

Sometimes, WebP icons won't load despite extensions.json clearly
defining it as the only extension used for all image data.

I suspect there's a race condition between fetching extensions.json,
parsing it into client, and checking what extension we should use to get
character icons during loading. Sometimes it correctly loads images,
sometimes it falls back and starts requesting PNG instead.

I couldn't precisely identify where it happens and what's the root
cause. As a workaround, this commit instead makes WebP the
first-priority extension and a fallback.

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