var number = 1;

function add_file_upload(){

number++;
document.getElementById('number').value = number;
var table = document.getElementById("files");
var row = table.insertRow(-1);
var group = row.insertCell(0);
group.id = "file";
var file = row.insertCell(1);
file.id = "file";
group.innerHTML = '<input type="text" name="group' + number +'" value="default">';
file.innerHTML = '<input type="file" name="file' + number +'" required >';

}