Rails Mailer issue with sending attchemnt over mail

I am migrating an application from Rails 2.3 to Rails 3.1, the emails are not working, when i send an email with attachment i see a plain/text email with the encoded pdf content in the email instead of as an attachment.

I see the content as

--
Date: Mon, 10 Aug 2015 16:16:26 +0530
Mime-Version: 1.0
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit
Content-ID: <55c881028af96_48d43fe94782e9a0658aa@amol-desktop.mail>

Please follow the instructions within the forms to return your merchandise. Thank You.

--
Date: Mon, 10 Aug 2015 16:16:26 +0530
Mime-Version: 1.0
Content-Type: application/pdf;
 charset=UTF-8
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename=free_book.pdf
Content-ID: <55c88102872ba_48d43fe94782e9a0657a0@amol-desktop.mail>

JVBERi0xLjQKJcfsj6IKNyAwIG9iago8PC9MZW5ndGggOCAwIFIvRmlsdGVy
IC9GbGF0ZURlY29kZT4+CnN0cmVhbQp4nH1W227bOBB991cM0AWqABZLUvc8
bTZNivTiuLUX2GKzD6xE22pk0aWktOnX75CSbUW+xEhkDufMGc4cjvIDKGFA
zad7puvRmy8JLKuRtYJejnyfghd5MQRBSIGjTY4W1srDkB+xeh4HHocQ92yM
clxzr7NFLD6w7fx6WOtn4gWhD2ewjEfeaZIeuHW0GUYxg3Nwn/rhGaI+vnVt
8wzhHN4PqX+GqQ+3njbTxIez8DhOzhD14dZzUM+j8MD3zxH14K3nIOZReEjp

here is the code that i use to send attachments

load_smtp_settings("noreply")
@recipients = email
@subject    = "#{name}: hello"
@body       = "Please follow the instructions within"
attachments['free_book.pdf'] = {mime_type: 'application/pdf',content:  File.read(pdf_document_path) }
@from       = email
@date    = Time.zone.now
@headers    = {}