I solved this by extending the grocery crud class and implementing this method:
protected function get_upload_file_readonly_input($field_info, $value)
{
if(empty($value))
return '';
//Fancybox
$this->load_js_fancybox();
$this->set_js_config($this->default_javascript_path.'/jquery_plugins/config/jquery.fancybox.config.js');
$unique = mt_rand();
$is_image = !empty($value) &&
( substr($value,-4) == '.jpg'
|| substr($value,-4) == '.png'
|| substr($value,-5) == '.jpeg'
|| substr($value,-4) == '.gif'
|| substr($value,-5) == '.tiff')
? true : false;
$image_class = $is_image ? 'image-thumbnail' : '';
$file_url = base_url().$field_info->extras->upload_path.'/'.$value;
$input = "<a href='".$file_url."' id='file_$unique' class='open-file";
$input .= $is_image ? " $image_class'><img src='".$file_url."' height='50px'>" : "' target='_blank'>$value";
$input .= "</a> ";
return $input;
}