How can i resend the same file with the uploader without asking the user to select another file and then reselect the original file ?
Yes, you can. You should go through the following steps:
Get the ID of an uploaded file (the one you want to resend) and its object:
//provided that you upload 1 file at a time
var id = $$("my_uploader").files.getFirstId();
var item = $$("my_uploader").files.getItem(id);
Change file status (it won’t be sent to server unless the status is client)
item.status = "client";
$$("my_uploader").files.updateItem(id, item);
Then you can send it
$$("my_uploader").send(); //or send(id);
Make sure you server script will properly handle the upload of two same files.