How to get url and description of FileFieldItemList's items in node template
I am able to access the uri, and I can generate url from it by a Twig filter.

{{ content.field_document.0['#file'].uri.value|urlFromUri }}
{{ content.field_document.1['#file'].uri.value|urlFromUri }}
{{ content.field_document.2['#file'].uri.value|urlFromUri }}


and the description:

{{ content.field_document.0['#description'] }}
{{ content.field_document.1['#description'] }}
{{ content.field_document.2['#description'] }}


the Twig filter:

&lt;?php

namespace Drupal\something_general\TwigExtension;

use Drupal\Core\Url;


class Custom extends \Twig_Extension
{

  /**
   * Generates a list of all Twig filters that this extension defines.
   */
  public function getFilters()
  {
    return [
      new \Twig_SimpleFilter('urlFromUri', array($this, 'UrlFromUri')),
    ];
  }


  /**
   * Gets a unique identifier for this Twig extension.
   */
  public function getName()
  {
    return 'custom.something_general.twig_extension';
  }


  /**
   * @param $uri
   */
  public static function UrlFromUri($uri)
  {
    $realpath = drupal_realpath($uri);
    $path = str_replace($_SERVER['DOCUMENT_ROOT'], '', $realpath);

    return $path;
  }

}


I am also able, to get the number of the items of the FileFieldItemList:

{% for index in 1..node.field_document.getValue|length %}
{% endfor %}


And finally the iteration in the node--publication.html.twig file:

{% for index in 1..node.field_document.getValue|length %}
  {% set document = content.field_document[ (index-1) ] %}

  {{ dump(document['#file'].uri.value|urlFromUri) }}
  {{ dump(document['#description']) }}

{% endfor %}