doc.rb

class Doc < File
  has_attachment :content_type => 'application/msword',
                 :max_size      => 2.megabytes,
                 :storage       => :file_system
end

file.rb

class File < ActiveRecord::Base
  belongs_to :invoice
end

invoice.rb

class Invoice < ActiveRecord::Base
  has_one :pdf
  has_one :doc
end

invoices_controller.rb

class InvoicesController

  def new
    @invoice = Invoice.new
    @file = File.new
  end

end

pdf.rb

class Pdf < File
  has_attachment :content_type => 'application/pdf',
                 :max_size      => 2.megabytes,
                 :storage       => :file_system
end

xyz_create_files.rb

class CreateFiles < ActiveRecord::Migration
  def self.up
    create_table :files do |t|
      t.string  :type, :file_name, :content_type
      t.integer :size, :invoice_id
      t.timestamps
    end
  end

  def self.down
    drop_table :addresses
  end
end