Yes, you see the field, but not the button. I think it's because of the fact that there's already an upload button on the form.
It is certainly the same problem as for the submit button, you can only have one by form.
Leonid to explain here how to do it using the mode "developer": https://una.io/page/view-discussion?id=808
But I admit that I did not manage to do it because I do not know what the serialized tables are.
Leonid, is it the same?