var form_data = new FormData(form);
Das FormData-Dingenkirchens taugt nur eingeschränkt.
Nutze es anders, also die append-Methode:
var pullfiles = function(form_data_object){
var fileInput = document.querySelector("#myfiles");
var files = fileInput.files;
var fl = files.length;
var i = 0;
var size = 0;
while ( i < fl) {
var file = files[i]; //
var nr = i + 1;
var name = 'file_nr_' + nr;
form_data_object.append(name, file, name);
i++;
}
// weiter mit dem form_data_object
}
// input type file multipart
document.querySelector("#myfiles").onchange = pullfiles;
Ungetestet.