def self.max_allowed_packet
    r = connection.execute "SHOW GLOBAL VARIABLES LIKE 'max_allowed_packet'"
    r.fetch_row.last.to_i  rescue 128.megabytes
  end
  validates_length_of :data, :in => 1 .. max_allowed_packet,
    :allow_blank => true
  validates_length_of :file, :in => 1 .. max_allowed_packet,
    :allow_blank => true