(Based on Programmatically trigger "select file" dialog box)
You call the hidden input button from the dat.GUI button's function.
<input id="myInput" type="file" style="visibility:hidden" />
<script>
var params = {
loadFile : function() {
document.getElementById('myInput').click();
}
};
var gui = new dat.GUI();
gui.add(params, 'loadFile').name('Load CSV file');
</script>