Postman error using sellsign API

Hey guys, I’m new to Postman and I’m using it to test the API sellsign, it’s about sending a pdf to a customer, so he can sign it by e-mail.
My error is : Cannot command packet from request : no json content found in application/octet-stream - test.sellsign:test.sellsign (1317); application/pdf - Test Signature.pdf:Test Signature.pdf (151381);

I use a multipart form data I guess :

The first file contains my parameters and the second it’s the pdf to sign.

I have the impression that postman can’t access my files.

Tell me if you need more informations, (and sorry my english is a little bit rusty).