Rails 4, Mailer preview, preview attached images

Question

I have problem with rails 4.1.6 mailer preview.

I want to see attached images in preview mode, but it doesn't work. I think it's incorrect

There is my code:

Mailer file

class AppMailer < ActionMailer::Base
  default from: "[email protected]"

  # AppMailer.test_mail.deliver
  def test_mail
    attachments.inline['rails.png'] = File.read(Rails.root.join('app', 'assets', 'images', 'rails.png'))

    mail(
      to: '[email protected]',
      subject: "test letter",
      template_path: "mailers",
      template_name: "test"
    )
  end
end

Preview file

class MailerPreview < ActionMailer::Preview
  def app_mailer
    AppMailer.test_mail
  end
end

Template file

%p Hello, World!
%p= image_tag attachments['rails.png'].url

When I go to

/rails/mailers/mailer/app_mailer

I see preview page, but images doesn't work. There is resulted html code

<p>Hello, World!</p>
<p><img src="cid:[email protected]"></p>

So. I think, I should to find a way to get path/to/file instead CID in preview mode

(When I sent letter to my mailbox - letter looks fine)

What I am doing wrong in preview mode?

Answer 1

For Rails >= 4.2 to preview images you should create initializer:

# config/initializer/preview_interceptors.rb
ActionMailer::Base.register_preview_interceptor(ActionMailer::InlinePreviewInterceptor)

Answer 2

You're not doing anything wrong; it's a shortcoming in the way the Rails Mail Previewer is designed.

The Rails Mailer code, reasonably, generates URLs for attachments that reference mail "parts" in a multipart email. The "cid" in the URL for your <img> tag refers to the "content ID" of the particular part/attachment. This is how URLs within an email work.

However, the previewer controller isn't rendering in the context of an email client: it's a standard web browser. There is no "cid" URL protocol scheme, and no multi-part email to reference (it's all just standard HTML documents). Rails::MailersController currently isn't smart enough to realize this, and just renders the email as-is.

To make this work, it'd have to detect all references to cid: URLs and replace them with regular http: URLs back to itself, then return the content for the various attachments.

There's an open issue on GitHub/rails to do this, but as of yet it's not complete.