attachment.rb

# The simplest thing that could possibly work.
class Attachment < ActiveRecord::Base
  before_create :store_metadata
  after_create :store_file

  # Virtual attribute that stores the uploaded tempfile
  attr_accessor :uploaded_file

  private
  
  def store_metadata
    self.size = uploaded_file.size
    self.content_type = uploaded_file.content_type
  end

  def store_file
    File.open(file_storage_location, "w") do |f|
      f.write uploaded_file.read
    end
  end

  def file_storage_location
    File.join(Rails.root, 'public', 'attachments', uploaded_file.original_filename)
  end
end

the_view.html.erb

<% form_for @attachment, :multipart => true do |f| %>
  <%= f.file_field :uploaded_file %>
  <%= f.submit "Upload file" %>
<% end %>