demo_app/my_platform/db/migrate/20150325105860_add_content_type_and_file_size_to_attachments.your_platform.rb
# This migration comes from your_platform (originally 20120815205610)
class AddContentTypeAndFileSizeToAttachments < ActiveRecord::Migration[4.2]
def change
add_column :attachments, :content_type, :string
add_column :attachments, :file_size, :integer
end
end