WP_REST_Attachments_Controller::create_item_permissions_check( WP_REST_Request $request )

	public function create_item_permissions_check( $request ) {
		$ret = parent::create_item_permissions_check( $request );

		if ( ! $ret || is_wp_error( $ret ) ) {
			return $ret;
		}

		if ( ! current_user_can( 'upload_files' ) ) {
			return new WP_Error( 'rest_cannot_create', __( 'Sorry, you are not allowed to upload media on this site.' ), array( 'status' => 400 ) );
		}

		// Attaching media to a post requires ability to edit said post.
		if ( ! empty( $request['post'] ) ) {
			$parent = get_post( (int) $request['post'] );
			$post_parent_type = get_post_type_object( $parent->post_type );

			if ( ! current_user_can( $post_parent_type->cap->edit_post, $request['post'] ) ) {
				return new WP_Error( 'rest_cannot_edit', __( 'Sorry, you are not allowed to upload media to this post.' ), array( 'status' => rest_authorization_required_code() ) );
			}
		}

		return true;
	}