[issue20197] Support WebP image format detection in imghdr module

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

LGTM. Detecting function matches the WebP specification [1].

[1] https://developers.google.com/speed/webp/docs/riff_container#webp-file-header

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<http://bugs.python.org/issue20197>
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