Source

File: wp-admin/includes/class-custom-image-header.php

public function get_uploaded_header_images() {
		$header_images = get_uploaded_header_images();
		$timestamp_key = '_wp_attachment_custom_header_last_used_' . get_stylesheet();
		$alt_text_key  = '_wp_attachment_image_alt';

		foreach ( $header_images as &$header_image ) {
			$header_meta               = get_post_meta( $header_image['attachment_id'] );
			$header_image['timestamp'] = isset( $header_meta[ $timestamp_key ] ) ? $header_meta[ $timestamp_key ] : '';
			$header_image['alt_text']  = isset( $header_meta[ $alt_text_key ] ) ? $header_meta[ $alt_text_key ] : '';
		}

		return $header_images;
	}