Spam filters often require that an HTML email also have a text alternative that is generally the same as the HTML message. This means you need to send an email with a MIME type of multipart/alternative containing text/html and text/plain parts. ActionMailer 3 supports this scenario, but it falls apart when you need to add (inline) attachments to that mix. The proper MIME hierarchy for an email like this is:

• multipart/mixed
  • multipart/alternative
    • multipart/related
      • text/html
      • image/png (e.g. for an inline attachment; pdf would be another good example)
    • text/plain
  • application/zip (e.g for an attachment--not inline)

If this seems more complicated then it should be, that's because it is. Thankfully, this gem allows you to create this entire hierarchy without all the hard work.

Either include

gem "mail_alternatives_with_attachments"

in your Gemfile (if using Bundler) or run

gem install mail_alternatives_with_attachments

Including the mail_alternatives_with_attachments gem in your project will patch ActionMailer with the following two methods:

• ActionMailer::Base#prepare_message(headers={}): This method does exactly what ActionMailer::Base#mail(headers={}) does; however it doesn't automatically render templates. This allows us to add our own custom message parts.
• Mail::Message#alternative_content_types_with_attachment(options, &block): This method allows you to conveniently add all of the different parts of a complex email with alternatives and attachments.

Typically when using ActionMailer 3, you would create a message with the following code:

class MyEmailerClass < ActionMailer::Base
  def my_email_method(address)
    mail :to => address, 
         :from => "noreply@myemail.com",
         :subject => "My Subject"
  end
end

To create an email with both alternatives and attachments you would use the following code:

class MyEmailerClass < ActionMailer::Base
  def my_email_method(address, attachment, logo)
    message = prepare_message to: address, subject: "My Subject", :content_type => "multipart/mixed"
    
    message.alternative_content_types_with_attachment(
      :text => render_to_string(:template => "my_template.text"),
      :html => render_to_string(:template => "my_template.html")
    ) do |inline_attachments|
      inline_attachments.inline['logo.png'] = logo
    end
    
    attachments['attachment.pdf'] = attachment
    
    message
  end
end