Running diagnose sniffer packet on Fortinet Fortigate unit outputs human-readable packet information and packet data . Only that sometimes you would like to have the traffic sniffed at Fortigate in Wireshark-readable format so that it can be analyzed by all powerful Wireshark.
For this case Fortinet came up with the script and application that takes text output of this sniffer command and parses it into tcpdump format (.cap) which you can later open in Wireshark.
I guess there are other scripts available that do just that (after all it is just parsing the text file) , but from Fortinet you can find it here:
kb.fortinet.com/kb/viewContent.do?externalId=11186&sliceId=1

Or by searching their website for
fgt2eth.pl
fgt2eth.zip