Ok we use it with this code :
<?php if ((!empty($this->item->jcfields[11]->value)) && (!empty($this->item->jcfields[12]->value))) : ?>
<div class="lien"><a href="/<?php echo $this->item->jcfields[11]->value; ?>" title="download file"><?php echo $this->item->jcfields[12]->value; ?></a> </div>
<?php endif ; ?>
where id 11 is eldownload and id 12 a simple text field
But now we have a serious issue : when we change the file in the fields the results en frontend never change. The simple text change in front if we change it in backoffice. If it can help you, if we show the plain link instead of href, we get a link with the name of the changed/new selected file BUT the files downloded is still the old one. As if the token or whatever stick to the first file we select.