using Paperclip for multiple attachments and displaying them using the method described in Advanced Rails Recipe #13

_asset.html.erb

<div class="asset"> 
  <% new_or_existing = asset.new_record? ? 'new' : 'existing' %>
  <% prefix = "page[#{new_or_existing}_asset_attributes][]" %>
  
  <% fields_for prefix, asset do |asset_form| -%> 
    <p> 
      Asset: <%= asset_form.file_field :file %> 
      <%= link_to_function "remove", "$(this).up('.asset').remove()" %> 
    </p> 
<% end -%> 
</div>

_form.html.erb

<%= error_messages_for :page %>

<% form_for @page, :html => { :multipart => true } do |f| %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.label :permalink %><br />
    <%= f.text_field :permalink %>
  </p>
  <p>
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  </p>
  <div id="assets">
    Attach a file or image<br />
    <%= render :partial => 'asset', :collection => @page.assets %> 
  </div> 
  <p> 
    <%= add_asset_link "Add a file" %> 
  </p>
  <p>
    <%= f.submit button_name %>
  </p>
<% end %>

<%= link_to 'Cancel', pages_path %>

asset.rb

class Asset < ActiveRecord::Base
  belongs_to :page
  
  has_attached_file :file, :styles => { :medium => "300x300>", :thumb => "100x100>" }
end

edit.html.erb

<!-- new.html.erb -->
<h1>New page</h1>

<%= render :partial => 'form', :locals => { :page => @page, :button_name => 'Create' } %>

<!-- edit.html.erb -->
<h1>Editing page</h1>

<%= render :partial => 'form', :locals => { :page => @page, :button_name => 'Update' } %>

index.html.erb

<div class="side-nav actions">
  <%= link_to "New", new_page_path if logged_in? %>
</div>

<%= title('Pages', :h1)%>

<div id="pages">
<% for page in @pages %>
  <div class="page">
<%= render :partial => 'shared/side_nav', :locals => { :obj => page, :edit_obj => edit_page_path(page) } %>
    <div class="main">
      <h2><%= page.name %></h2>
      <div class="content"><%= truncate(page.content) %></div>
    </div>
  </div>
<% end %>
</div>

page.rb

class Page < ActiveRecord::Base 
  has_many :assets, :dependent => :destroy
  
  validates_associated :assets
  validates_presence_of :name, :permalink
  
  after_update :save_assets
  
  def new_asset_attributes=(asset_attributes) 
    asset_attributes.each do |attributes| 
      assets.build(attributes) 
    end 
  end
  
  def existing_asset_attributes=(asset_attributes) 
    assets.reject(&:new_record?).each do |asset| 
      attributes = asset_attributes[asset.id.to_s] 
      if attributes 
        asset.attributes = attributes 
      else 
        asset.delete(asset) 
      end 
    end 
  end 
  
  def save_assets 
    assets.each do |asset| 
      asset.save(false) 
    end 
  end 
  
end

pages_controller.rb

class PagesController < ApplicationController
  before_filter :login_required, :except => [ :show ]
  
  # GET /pages
  # GET /pages.xml
  def index
    @pages = Page.find(:all)

    respond_to do |format|
      format.html # index.html.erb
      format.xml  { render :xml => @pages }
    end
  end

  # GET /pages/1
  # GET /pages/1.xml
  def show
    if params[:permalink]
      @page = Page.find_by_permalink(params[:permalink])
      raise ActiveRecord::RecordNotFound, "Page not found." if @page.nil?
      respond_to do |format|
        format.html # show.html.erb
        format.xml  { render :xml => @page }
      end
    else
      @page = Page.find(params[:id])
      respond_to do |format|
        format.html # show.html.erb
        format.xml  { render :xml => @page }
      end
    end  
  end

  # GET /pages/new
  # GET /pages/new.xml
  def new
    @page = Page.new
    @page.assets.build

    respond_to do |format|
      format.html # new.html.erb
      format.xml  { render :xml => @page }
    end
  end

  # GET /pages/1/edit
  def edit
    @page = Page.find(params[:id])
  end

  # POST /pages
  # POST /pages.xml
  def create
    @page = Page.new(params[:page])

    respond_to do |format|
      if @page.save
        flash[:notice] = 'Page was successfully created.'
        format.html { redirect_to(@page) }
        format.xml  { render :xml => @page, :status => :created, :location => @page }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @page.errors, :status => :unprocessable_entity }
      end
    end
  end

  # PUT /pages/1
  # PUT /pages/1.xml
  def update
    params[:page][:existing_asset_attributes] ||= {}
    
    @page = Page.find(params[:id])

    respond_to do |format|
      if @page.update_attributes(params[:page])
        flash[:notice] = 'Page was successfully updated.'
        format.html { redirect_to(@page) }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @page.errors, :status => :unprocessable_entity }
      end
    end
  end

  # DELETE /pages/1
  # DELETE /pages/1.xml
  def destroy
    @page = Page.find(params[:id])
    @page.destroy

    respond_to do |format|
      format.html { redirect_to(pages_url) }
      format.xml  { head :ok }
    end
  end
end

pages_helper.erb

def add_asset_link(name) 
    link_to_function name do |page| 
      page.insert_html :bottom, :assets, :partial => 'asset', :object => Asset.new 
    end 
  end

schema.rb

  create_table "assets", :force => true do |t|
    t.string   "name"
    t.text     "description"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.string   "file_file_name"
    t.string   "file_content_type"
    t.integer  "file_file_size"
    t.datetime "file_updated_at"
    t.integer  "page_id"
  end

  create_table "pages", :force => true do |t|
    t.string   "name"
    t.string   "permalink"
    t.text     "content"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

show.html.erb

<%= render :partial => 'shared/show_side_nav', :locals => { :obj => @event, :edit_obj => edit_page_path(@page) } %>

<%= title(@page.name, :h1) %>

<%= simple_format h(@page.content) %>

<% for asset in @page.assets %>
  <p><%= image_tag asset.file.url %></p>
  <p>Small:<%= image_tag asset.file.url(:medium) %></p>
<% end %>

Looks good, but missing the strong parameters in the PageController. Also, link_to_function is being deprecated/removed (or already is in 4.1?), and it should be asset.save(:validate => false).