max_file_size()
  Realscanner to test email's size.
  
  Usage: max_file_size(size, name='FileSizeOverrun')
  
  Where: size is a number of bytes, which if exceeded, virus is returned
         name is a string, which identifies the virus name.
            If this parameter is not present, 'FileSizeOverrun' is returned.
  
  Example: max_file_size(1024*1024*10)
           # all files at least 10MB will be reported as virus