Wrapper class to enable using AWS S3 objects as attachments when sending mail via the Mailgun REST interface using RestClient. This allows reading directly from the S3 object instead of creating an intermediate temp file.

MailgunS3Attachment.rb

#
# This class will wrap an S3 object and provide the methods that the Mailgun library expects in order
# to use the S3 object for sending a mail attachment.
#
# Use as follows:
#
# data = Multimap.new
# data[:from] = "My Self <no-reply@example.com>"
# data[:subject] = "Subject"
# data[:to] = "#{recipients.join(', ')}"
# data[:text] = text_version_of_your_email
# data[:html] = html_version_of_your_email
# data[:attachment] = MailgunS3Attachment.new(file_name, file_key)
# RestClient.post "https://api:#{API_KEY}"\
#   "@api.mailgun.net/v2/#{MAILGUN_DOMAIN}/messages",
#   data
#
# Note that some environment variables are expected to be set; you can change these to config vars in your
# app if you want to, of course. See #initialize for those.
#
# One other note: This unfortunately has to read in the complete file in order to be able to hand out chunks
# of the S3 object. If your attachments are really big, this could be a Bad Thing so be aware of it.
#
class MailgunS3Attachment

  # file_name: the name that you want the attachment to have when the recipient receives the email
  # file_key: the S3 object in your bucket
  def initialize(file_name, file_key)
    @file_name = file_name
    @file_key = file_key
    @contents = nil
    @offset = 0
    s3 = AWS::S3.new(access_key_id: ENV['AWS_ACCESS_KEY_ID'], secret_access_key: ENV['AWS_SECRET_KEY_ID'])
    b = s3.buckets[ENV['AWS_BUCKET']]
    @s3_file = b.objects[file_key]
  end

  # This provides the read method that the Mailgun library wants. It will call read(8142) until it receives nil,
  # so this code handles a crude sort of buffering by reading the file in the first time and then handing out the
  # requested pieces of it. Could be worse I guess.
  def read(length = nil)
    @contents = @s3_file.read.force_encoding('utf-8') if @offset == 0  # read if we haven't yet read anything; this returns a String
    if (length.nil?)
      return @contents
    else
      old_offset = @offset
      @offset += length
      return @contents[old_offset..(@offset-1)]
    end
  end

  def close
  end

  def path
    @file_key
  end

  # This ensures that the attachment has the name you want, instead of the S3 file_key.
  def original_filename
    @file_name
  end

  # The content_type does matter for Mailgun attachments! If you try to send an msword attachment with
  # content_type of 'text/plain', it will come through corrupted in the email. No idea why.
  def content_type
    case @file_name.split('.').last
    when 'doc'
      'application/msword'
    when 'pdf'
      'application/pdf'
    when 'rtf'
      'application/rtf'
    else
      'text/plain'
    end
  end

end