var nodemailer = require("nodemailer");

var smtpTransport = nodemailer.createTransport("SMTP",{
   service: "Gmail",
   auth: {
       user: "gmail.user@gmail.com",
       pass: "gmailpass"
   }
});

smtpTransport.sendMail({
   from: "My Name <me@example.com>", // sender address
   to: "Your Name <you@example.com>", // comma separated list of receivers
   subject: "Hello ✔", // Subject line
   text: "Hello world ✔" // plaintext body
}, function(error, response){
   if(error){
       console.log(error);
   }else{
       console.log("Message sent: " + response.message);
   }
});

<form id="mainForm">
    <input type="file" id="fileUpload">
    <input type="submit" id="submit" name="submit">
</form>

var app = express();
var Busboy = require('busboy');
var nodemailer = require("nodemailer");

var smtpTransport = nodemailer.createTransport("SMTP",{
    service: "Gmail",
     auth: {
         user: "gmail.user@gmail.com",
         pass: "gmailpass"
     }
});

app.post('/email', function(req, res){
    var busboy = new Busboy({ headers: req.headers });
    var attachments = [];

    var mailOptions = {
        from: "My Name <me@example.com>", // sender address
        to: "Your Name <you@example.com>", // comma separated list of receivers
        subject: "Hello ✔", // Subject line
        text: "Hello world ✔" // plaintext body
    };

    busboy
        .on('file', function(fieldname, file, filename, encoding, mimetype){
            attachments.push({
               filename: filename,
               content: file.toString('base64'),
               encoding: 'base64'
            });
        })
        .on('finish', function() {
            mailOptions.attachments = attachments;
            smtpTransport.sendMail(mailOptions, function (err, info) {
               if (err) {
                   //handle error
               }
                // email sent
           });
        });
});