Filtering the WP REST API endpoints.

rest-endpoints-filter.php

<?php 
add_action( 'rest_endpoints', function( $endpoints ){
    if( isset( $endpoints[ '/wp/v2/posts' ] ) ){
        foreach( $endpoints[ '/wp/v2/posts' ] as &$post_endpoint ){            
            if( ! empty( $post_endpoint[ 'methods' ] ) && 'GET' == $post_endpoint[ 'methods' ] ){
                $post_endpoint[ 'args' ][ 'type' ] = array(
                    'description' => 'Post types',
                    'type' => 'array',
                    'required' => false,
                    'default' => 'post'
                );
            }
        }
    }
    return $endpoints;
}, 15 );