Add attachment to email with SendGrid using rails

I've created a hello_world method which sends an email with Sendgrid. I am trying to include an attachment. I've found the following line in another stackoverflow answer: mail.attachments['test.txt'] = File.read("#{Rails.root}/public/test.txt")

This however generates the following error:

Completed 500 Internal Server Error in 17ms (ActiveRecord: 3.4ms)



TypeError - no implicit conversion of String into Integer:
  app/controllers/messages_controller.rb:32:in `hello_world'
  app/controllers/messages_controller.rb:65:in `create'

Mailing code in controller:

  def hello_world(company, message)
    from = Email.new(email: "test+#{current_user.auth_token}@example.com")
    to = Email.new(email: 'hello@pim.gg')

    subject = 'TEST from dev'
      content = Content.new(type: 'text/plain', value: "#{company.email} #{current_user} #{current_user.first_name} #{current_user.last_name} #{message.text}")

    mail = SendGrid::Mail.new(from, subject, to, content)
    mail.attachments['test.txt'] = File.read("#{Rails.root}/public/test.txt")

    sg = SendGrid::API.new(api_key: ENV['SENDGRID_API_KEY'])
    response = sg.client.mail._('send').post(request_body: mail.to_json)
    puts response.status_code
    puts response.body
    puts response.headers
  end

According to documentation of sendgrid-ruby gem adding-attachments should be like this:

attachment = SendGrid::Attachment.new
attachment.content = Base64.strict_encode64(File.open(fpath, 'rb').read)
attachment.type = 'application/vnd.openxmlformatsofficedocument.spreadsheetml.sheet'
attachment.filename = fname
attachment.disposition = 'attachment'
attachment.content_id = 'Reports Sheet'
mail.add_attachment(attachment)