Hello ! i'm trying to do the exact same thing but so far I found only dead ends...
1) Tried to get the id of the ir.attachments that correspond to the payslip documents.documents and then xmlrpc searching for the fields datas and raw, but I couldn't get the ir.attachment call to return something, even though I had the id from searching on documents...
2) tried post requesting directly the url but since there is 2FA authentication it's imposs to authenticatz through a script
3) tried xmlrpc calling the http method directly but only got error 303 and some cookie set to nosniff.
Did you find any solutions ?