Using tshark to decrypt tls/ssl.

edit

Wireshark doesn't save a decrypted file but you can add the decryption keys to the capture file:
TLS\SSL pcap with key - save decrypted output to pcap file without the attach key
To decrypt with tshark, set the -o tls.keylog_file:<filename> preference.
Also check the Wireshark wiki TLS page for links to presentations using tshark with TLS.

ask_wireshark$ tshark -r ./tls_pcap.pcapng -z io,phs,tls -q

===================================================================
Protocol Hierarchy Statistics
Filter: tls

frame                                    frames:11074 bytes:14792926
  eth                                    frames:11074 bytes:14792926
    ip                                   frames:11074 bytes:14792926
      tcp                                frames:11074 bytes:14792926
        tls                              frames:11074 bytes:14792926
          tcp.segments                   frames:1087 bytes:1457505
            tls                          frames:1073 bytes:1446693
            data                         frames:4 bytes:5416
===================================================================
ask_wireshark$ tshark -r ./tls_pcap.pcapng -z io,phs,tls -o tls.keylog_file:keys.txt -q

===================================================================
Protocol Hierarchy Statistics
Filter: tls

frame                                    frames:11074 bytes:14792926
  eth                                    frames:11074 bytes:14792926
    ip                                   frames:11074 bytes:14792926
      tcp                                frames:11074 bytes:14792926
        tls                              frames:11074 bytes:14792926
          tcp.segments                   frames:1077 bytes:1445986
            tls                          frames:1065 bytes:1435861
            data                         frames:4 bytes:5416
          http2                          frames:32 bytes:5807
            xml                          frames:1 bytes:134
          http                           frames:7 bytes:3271
            json                         frames:1 bytes:454
              tcp.segments               frames:1 bytes:454
<snip>