Mailgun messages.mime returning 400 with content length header

index.js

'use strict'

const nodemailer = require('nodemailer')
const got = require('got')
const FormData = require('form-data')

class MailGunTransporter {
  send (mail, callback) {
    const form = new FormData()
    form.append('to', '<TO_EMAIL>')
    form.append('message', mail.message.createReadStream(), { filename: 'message.txt' })

    const req = got('https://api.mailgun.net/v3/<YOUR_DOMAIN>/messages.mime', {
      body: form,
      stream: true,
      auth: 'api:<API_KEY>'
    })

    req.on('request', (req) => {
      // Uncommenting the following line will send the email through

      // req.removeHeader('content-length')
    })

    req.on('end', () => {
      callback(null)
    })

    req.on('error', (error) => {
      callback(error)
    })
  }
}

const transporter = nodemailer.createTransport(new MailGunTransporter())

transporter.sendMail({
  from: '<FROM_EMAIL>',
  to: '<TO_EMAIL>',
  subject: 'Hi guys',
  text: 'hello world'
}, function (error) {
  console.log(error)
})

MIME

Content-Type: text/plain
From: <FROM_EMAIL>
To: <TO_EMAIL>
Subject: Hi guys
Message-ID: <522def0e-b93f-5b93-60ee-8a08924af4d0@dimerapp.com>
Content-Transfer-Encoding: 7bit
Date: Tue, 08 May 2018 08:50:54 +0000
MIME-Version: 1.0

hello world

Curl request

 curl --user 'api:<API_KEY>' \
    https://api.mailgun.net/v3/<DOMAIN>/messages.mime \
    -H "Content-length:300" \
    -F to='<TO>' \
    -F message=@email.mime