Certificate update via API

Hello everyone,

I am trying to add (and eventually update) a certificate on our FortiWeb appliance by using the API. I have read in the documentation that something like this:

/usr/bin/curl -i -X POST -H 'Authorization: YWRtaW46' -H "Content-Type: application/json" -k -d @test.json https://192.168.118.41:90/api/v1.0/System/Certificates/Local; echo

is what I am supposed to do. The "test.json" file is a file containing a json object describing a certificate, the frame for which I have built using a GET request on the same resource:

/usr/bin/curl -iv -X GET -H 'Authorization: YWRtaW46' -H "Content-Type: application/json" -k https://192.168.118.41:90/api/v1.0/System/Certificates/Local; echo

I am getting an error 500 after the POST request. The error message is troubling, saying that "File is not an update file". My question is as follow:

What would be a "valid" update file and where can I find an example of a successful POST request for a new certificate?

I did not want to bump an old thread: https://forum.fortinet.com/tm.aspx?m=155127&tree=true