I used paper clip in my web application, I use this to make a new product:
<% semantic_form_for @product do |f| %>  
  <% f.inputs do %>  
    <%= f.input :title %>  
    <%= f.input :price %>  
    <%= f.file_field :photo %>
    <%= f.input :category , :include_blank => false %>  
  <% end %>  
  <%= f.buttons %>  
<% end %>
And this to show product:
<% semantic_form_for @product do |f| %>  
<%= image_tag @product.photo.url%>
  <% f.inputs do %>  
    <%= f.input :title %>  
    <%= f.input :price %>  
    <%= f.file_field :photo %>
    <%= f.input :category , :include_blank => false %>  
  <% end %>  
  <%= f.buttons %>  
<% end %>
And his is my product.rb:
class Product < ActiveRecord::Base
  validates_presence_of :title, :price
  validates_numericality_of :price
  validates_uniqueness_of :title
  has_attached_file :photo
  attr_accessible :name, :category_id, :price, :title, :photo
  belongs_to :category
  has_many :order_items
end
But after I upload the photo, it show my image path like this :
It seems that it can't upload the photo with this error:
No route matches "/photos/original/missing.png" with{:method=>:get}