Note: put flag into AFFCTF{} format.
Open the pcap file in wireshark. The first packets show ftp traffic, so lets filter for it. Packet 2771 shows a binary upload into a folder called STRICTLY_CONFIDENTIAL and the content starts with VimCrypt~03!. So lets dump the encrypted data out and search for the key.
While further investigating the pcap file I found unencrypted smtp traffic and in packet 1438 a message that contains the password is Horse Battery Staple Correct. First I started googling but it turned out that taking the message literally is the answer.
When opening the data dump in vim and using Horse Battery Staple Correct as password it successfully decrypts the message with the content I_Should_Have_Used_Safer_Connection_....
flag: AFFCTF{I_Should_Have_Used_Safer_Connection_...}