Some improvements

I have made some changes in your plugin code so now it supports:
* WordPress 3.0 version,
* Attachments files download with cURL and no with copy, because in some servers in is does not work.
* PHP 5.1 support – because error_get_last function does not exist before PHP 5.2

I think you can include these changes into the code of plugin.

replace this code:
copy($old_filename, $new_filename)
BY
download($old_filename, $new_filename)
AND
Insert this functions in any part of your plugin:

if (!function_exists('error_get_last')) {
	function error_get_last(){
		$__error_get_last_retval__ = array(
			'type'        => 'simulated',
			'message'       => 'simulated',
			'file'        => 'simulated',
			'line'        => 'simulated'
		);
		return $__error_get_last_retval__;
	}

}

function download($url,$path){
	$fp = fopen($path, 'w');

	$ch = curl_init($url);
	curl_setopt($ch, CURLOPT_FILE, $fp);

	$return = curl_exec($ch);

	curl_close($ch);
	fclose($fp);
	return $return;
}

if (!function_exists('set_post_thumbnail')){
	function set_post_thumbnail( $post, $thumbnail_id ) {
		$post = get_post( $post );
		$thumbnail_id = absint( $thumbnail_id );
		if ( $post && $thumbnail_id && get_post( $thumbnail_id ) ) {
			$thumbnail_html = wp_get_attachment_image( $thumbnail_id, 'thumbnail' );
			if ( ! empty( $thumbnail_html ) ) {
				return update_post_meta( $post->ID, '_thumbnail_id', $thumbnail_id );
			}
		}
		return false;
	}
}

https://www.ads-software.com/extend/plugins/fg-joomla-to-wordpress/