Automattic/jetpack — pull request #173 by pippercameron (2014-02-05 05:34:38+00:00)
Title: Allow _thumbnail_id to be modified in postmeta via the REST API.

_thumbnail_id is a protected meta field and not editable by default. This change whitelists _thumbnail_id in WPCOM_JSON_API_Post_Endpoint::is_metadata_public() via the filter rest_api_allowed_public_metadata making setting featured images via the REST API possible.