Wordpress class to upload files from external URL to media library.

FileUploader.php

<?php

namespace App;

use RuntimeException;

require_once(ABSPATH . '/wp-load.php');
require_once(ABSPATH . '/wp-admin/includes/image.php');
require_once(ABSPATH . '/wp-admin/includes/file.php');
require_once(ABSPATH . '/wp-admin/includes/media.php');

class FileUploader
{

	/** @var string */
	private $url;

	/**
	 * Set external URL.
	 * @param string $url
	 */
	public function __construct(string $url)
	{
		$this->url = $url;
	}

	/**
	 * Upload file.
	 * @param string|null $title
	 * @param array $meta
	 * @return int Attachment id.
	 * @throws RuntimeException
	 */
	public function upload(string $title = null, array $meta = []): int
	{
		$tmp = download_url($this->url);
		if (is_wp_error($tmp)) throw new RuntimeException($tmp->get_error_message(), $tmp->get_error_code());

		$filename = pathinfo($this->url, PATHINFO_FILENAME);
		$extension = pathinfo($this->url, PATHINFO_EXTENSION);

		if (!$extension) $extension = $this->getMimeExtension($tmp);
		if (!$extension) {
			@unlink($tmp);
			throw new RuntimeException('Unknown file extension.');
		}

		$imgId = media_handle_sideload([
			'name' => "$filename.$extension",
			'tmp_name' => $tmp,
		], 0, $title);
		@unlink($tmp);

		if (is_wp_error($imgId)) throw new RuntimeException($imgId->get_error_message(), $imgId->get_error_code());

		foreach ($meta as $key => $value) {
			update_post_meta($imgId, $key, $value);
		}

		return $imgId;
	}

	/**
	 * Get file extension by mime type.
	 * @param string $path
	 * @return string|null
	 */
	private function getMimeExtension(string $path): ?string
	{
		$mime = mime_content_type($path);
		$mime = is_string($mime) ? sanitize_mime_type($mime) : false;

		$mimeExts = array(
			'text/plain'=> 'txt',
			'text/csv' => 'csv',
			'application/msword' => 'doc',
			'image/jpg' => 'jpg',
			'image/jpeg' => 'jpeg',
			'image/gif' => 'gif',
			'image/png' => 'png',
			'video/mp4' => 'mp4'
		);

		if (isset($mimeExts[$mime])) return $mimeExts[$mime];
		return null;
	}
}