The SSL master keys can be logged by mitmproxy so that external programs can decrypt TLS connections both from and to the proxy. Key logging is enabled by setting the environment variable SSLKEYLOGFILE so that it points to a writable text file. Recent versions of WireShark can use these log files to decrypt packets. You can specify the key file path in WireShark via
Edit → Preferences → Protocols → SSL → (Pre)-Master-Secret log filename. Note that SSLKEYLOGFILE is respected by other programs as well, e.g. Firefox and Chrome. If this creates any issues, you can set MITMPROXY_SSLKEYLOGFILE alternatively.