right now the metadata obtained from DHT is saved to "the filename is name in metadata".torrent
this choice of name works for single-file torrents, and for simple multifile-torrents (the ones which have a single root-directory), but a torrent structure could be more complex. It seems that the only reasonable universal choice for the name of the .torrent file is "hash-info".torrent. If one wants, he could rename it to ubuntu.iso.torrent himself later.
I decided to use info hash hex encoded 40 characters plus ".torrent" as a filename. Thank you for follow-up.