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.