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The current pattern wouldn't be able to sniff the 'lossless' and
'extended' WebP formats.

Adjust the pattern to match the one described by [1].

[1] https://mimesniff.spec.whatwg.org/#matching-an-image-type-pattern

Bug: 889420
Change-Id: I4b5a73d4d9477502d58df7460b9494e6540a59f3
Reviewed-on: https://chromium-review.googlesource.com/1248782Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#595273}