ActionMailer - Adding an attachment from S3

Seems simple enough but I haven't been able to get it to work. The
files work fine from S3 on my web app, but when I email them out via
ActionMailer as attachments, see the code below, the files are
corrupt.

App Stack: rails 3, heroku, paperclip + s3

Here's the code:

    class UserMailer < ActionMailer::Base
    # Add Attachments if any
    if @comment.attachments.count > 0
      @comment.attachments.each do |a|
        require 'open-uri'
        open("#{Rails.root.to_s}/tmp/#{a.attachment_file_name}", "wb")
do |file|
          file << open(a.authenticated_url()).read
          attachments[a.attachment_file_name] =
File.read("#{Rails.root.to_s}/tmp/#{a.attachment_file_name}")
        end
      end
    end

    mail( :to => "#{XXXX}",
          :reply_to => "XXXXX>",
          :subject => "XXXXXX"
          )

a.authenticated_url() just gives me a URL to s3 to get the file (of
any type), I checked this, works fine. Something to do with the way
I'm saving the tempfile must be breaking the ActionMailer Attachment.

Any ideas?