Add 'application/zstd' media type for Zstandard.
Add 'application/zstd' media type for Zstandard.

Reference:
https://tools.ietf.org/html/rfc8478#section-6.1

Signed-off-by: Masanori Ogino <c49462fef669560e618607913e6c283f7627123c@gmail.com>

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