how to make use of nodejs single thread to read files and attach data

var mailer = require('nodemailer');
mailer.SMTP = {
    host: 'host.com', 
    port:587,
    use_authentication: true, 
    user: '[email protected]', 
    pass: 'xxxxxx'
};

Then read a file and send an email :

fs.readFile("./attachment.txt", function (err, data) {

    mailer.send_mail({       
        sender: '[email protected]',
        to: '[email protected]',
        subject: 'Attachment!',
        body: 'mail content...',
        attachments: [{'filename': 'attachment.txt', 'content': data}]
    }), function(err, success) {
        if (err) {
            // Handle error
        }

    }
});