I have finally found a working solution:
<?php
$file = get_field('dokumente_datei');
$url = $file['url'];
$image_id = $file['id'];
$image_url = wp_get_attachment_image($image_id,'medium');
if( $file ):
?>
<a href="<?php echo $file['url']; ?>">
<?php echo ($image_url); ?></a>
<?php endif; ?>
