yii\mail\MessageInterface::embed PHP Method

This method should be used when embedding images or other data in a message.

public embed ( string $fileName, array $options = [] ) : string
$fileName	string	file name.
$options	array	options for embed file. Valid options are: - fileName: name, which should be used to attach file. - contentType: attached file MIME type.
return	string	attachment CID.

    public function embed($fileName, array $options = []);

MessageInterface