attachment.rb

# The simplest thing that could possibly work. Will probably overwrite files if someone uploads
# a file that has the same file name as an already uploaded file. Requires the columns 'size' and
# 'content_type', see the 'store_file' method. You could obviously just remove those calls
# if you don't want to store the file size and the content type in the database.
class Attachment < ActiveRecord::Base
  after_create :store_file

  # Virtual attribute that stores the uploaded tempfile
  attr_accessor :uploaded_file

  private

  def store_file
    File.open(file_storage_location, "w") do |f|
      f.write upload.read
    end
    self.size = uploaded_file.size
    self.content_type = uploaded_file.content_type
  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 %>