Ruby on Rails

I am uploading a PDF file using paper clip. File get uploaded
successfully. If I try to view the file directly from file system it is
opening.
I have to open the file from application so I have written following
code.
View Code:-
<%= link_to 'View Attachment', {:action=>"show_attachment",
:id=>user.id} %>

Controller Code:-
def show_attachment
   @user=User.find(params[:id])
    send_data(@user.attachment.url, :type => "application/pdf",
:filename => "attachment.pdf", :disposition => 'attachment')
 end

Can anyone tell me what is the issue?
I can see the pdf file from the uploaded location.