post.rb

# post.rb
class Post < ActiveRecord::Base
  has_attached_file :upload

  validates_presence_of :title
end

# posts_controller.rb
data = grab_data_for(@post)
file = StringIO.new(data)
file.class.class_eval { attr_accessor :original_filename }
file.original_filename = @post.to_filename
@post.upload = file