Source

File: wp-includes/rest-api/class-wp-rest-request.php

protected function parse_body_params() {
		if ( $this->parsed_body ) {
			return;
		}

		$this->parsed_body = true;

		/*
		 * Check that we got URL-encoded. Treat a missing content-type as
		 * URL-encoded for maximum compatibility.
		 */
		$content_type = $this->get_content_type();

		if ( ! empty( $content_type ) && 'application/x-www-form-urlencoded' !== $content_type['value'] ) {
			return;
		}

		parse_str( $this->get_body(), $params );

		/*
		 * Add to the POST parameters stored internally. If a user has already
		 * set these manually (via `set_body_params`), don't override them.
		 */
		$this->params['POST'] = array_merge( $params, $this->params['POST'] );
	}