def valid_dimensions

    if file.path[-3,3] == 'swf'
      header = SwfFile::FlashFile.header file.path

      width = header.width
      height = header.height
    else
      dimensions = ImageSize.path(file.path).size
      width = dimensions[0]
      height = dimensions[1]
    end

    unless (width == 600 && height == 200) || (width == 300 && height == 250) || (width == 468 && height == 60) || (width == 728 && height == 90) || (width == 160 && height == 600 || (width == 240 && height == 400))
        errors.add :base, "error"
    end
  end