Undefined method `mime_type` in Mailer view when using attachments

Question

Undefined method `mime_type` in Mailer view when using attachments

I created a mailer in my Rails 4 application that works great when there are no email attachments. However, when I add the PDF attachment for emails, I get the following error when I try to preview the email ... / rails / mailers / donor _mailer / thankyou_mail_preview

NoMethodError in Rails::Mailers#preview

Showing /usr/local/rvm/gems/ruby-2.1.5@rails4/gems/railties-4.1.6/lib/rails/templates/rails/mailers/email.html.erb where line #95 raised:

<iframe seamless name="messageBody" src="?part=<%= Rack::Utils.escape(@part.mime_type) %>"></iframe>

However, I don't get this error or any problems at all when sending the emails themselves, only with the preview. Has anyone else experienced this and knows how to fix it / knows the direction I should be going. Could this lead to further problems with my email, or is it just a problem with the preview function?

My Mailer looks like this:

class DonorMailer < ActionMailer::Base

  default from: "user@gmail.com"

  def thankyou_email(computer)
    @computer  = computer
    @donor = computer.donor

    attachments["some.pdf"] = File.read("path/to/pdf/some.pdf")
    mail(to: @donor.donor_email, subject: "Thank you")
  end

end

and the template:

<!DOCTYPE html>
<html>
  <head>
     <meta content='text/html; charset=UTF-8' http-equiv='Content-Type' />
  </head>

  <body>

    <p>Email content here</p>

  </body>
</html>