The challenge is the following,
We are also given the file torrent.pcap. As this is a torrent challenge, I went to Wireshark and enabled the BitTorrent DHT Protocol (BT-DHT) by going to Analyze -> Enabled Protocol.
The overall packet capture looks like the following,
I applied the bt-dht filter, and looked through the packets, and saw that some contained info_hash.
The challenge only wants us to find the file name, and not reconstruct the file, so I knew that this info_hash information will be very important because it tells us the hash of the file. As hash is 68 61 73 68 in hex, I inputted this hex value into the Wireshark search to look for all packets that contained this hash information.
The first packet that contained info_hash was packet 79 with a hash value of 17d62de1495d4404f6fb385bdfd7ead5c897ea22
So I looked up 17d62de1495d4404f6fb385bdfd7ead5c897ea22 on Google, and saw that it corresponded to Awakened.2013.1080p.BluRay.X264-iNVANDRAREN.
The first packet that contained info_hash was packet 332 with a hash value of 17c1e42e811a83f12c697c21bed9c72b5cb3000d
This file corresponded to name: Zoo (2017) 720p WEB-DL x264 ESubs - MkvHub.Com.
I looked through a few more, and I was at packet 51080 which had a hash value of e2467cbf021192c241367b892230dc1e05c0580e.
I Googled this, and saw that it corresponded to ubuntu-19.10-desktop-amd64.iso from LinuxTracker.org.
Therefore, the flag is,
picoCTF{ubuntu-19.10-desktop-amd64.iso}