def destination_format
      format   = File.extname(destination)[1..-1] if destination
      format ||= self.format
      format ||= File.extname(source_path)[1..-1] if source_path

      format || DEFAULT_FORMAT
    end
def destination_format
      format   = File.extname(destination)[1..-1] if destination
      format ||= self.format
      format ||= File.extname(source_path)[1..-1] if source_path

      format || DEFAULT_FORMAT
    end
Determines the appropriate destination image format.