Source

File: wp-includes/class-wp-customize-manager.php

protected function prepare_starter_content_attachments( $attachments ) {
		$prepared_attachments = array();
		if ( empty( $attachments ) ) {
			return $prepared_attachments;
		}

		// Such is The WordPress Way.
		require_once ABSPATH . 'wp-admin/includes/file.php';
		require_once ABSPATH . 'wp-admin/includes/media.php';
		require_once ABSPATH . 'wp-admin/includes/image.php';

		foreach ( $attachments as $symbol => $attachment ) {

			// A file is required and URLs to files are not currently allowed.
			if ( empty( $attachment['file'] ) || preg_match( '#^https?://$#', $attachment['file'] ) ) {
				continue;
			}

			$file_path = null;
			if ( file_exists( $attachment['file'] ) ) {
				$file_path = $attachment['file']; // Could be absolute path to file in plugin.
			} elseif ( is_child_theme() && file_exists( get_stylesheet_directory() . '/' . $attachment['file'] ) ) {
				$file_path = get_stylesheet_directory() . '/' . $attachment['file'];
			} elseif ( file_exists( get_template_directory() . '/' . $attachment['file'] ) ) {
				$file_path = get_template_directory() . '/' . $attachment['file'];
			} else {
				continue;
			}
			$file_name = wp_basename( $attachment['file'] );

			// Skip file types that are not recognized.
			$checked_filetype = wp_check_filetype( $file_name );
			if ( empty( $checked_filetype['type'] ) ) {
				continue;
			}

			// Ensure post_name is set since not automatically derived from post_title for new auto-draft posts.
			if ( empty( $attachment['post_name'] ) ) {
				if ( ! empty( $attachment['post_title'] ) ) {
					$attachment['post_name'] = sanitize_title( $attachment['post_title'] );
				} else {
					$attachment['post_name'] = sanitize_title( preg_replace( '/\.\w+$/', '', $file_name ) );
				}
			}

			$attachment['file_name']         = $file_name;
			$attachment['file_path']         = $file_path;
			$prepared_attachments[ $symbol ] = $attachment;
		}
		return $prepared_attachments;
	}