Field twig template : override HTML output
You only need the twig content between &lt;ul&gt; ... &lt;/ul&gt;. The variables you set  are not used anywhere. And you don't need to extend the field template from the base theme to override it. Only if you want to replace twig blocks.

field--field-fichier-joint.html.twig

{%
  set classes = [
    'class1',
    'class2',
  ]
%}

&lt;ul{{ attributes.addClass(classes) }}&gt;
  {% for item in items %}
      {% set zebra = cycle(['odd', 'even'], loop.index0) %}
      &lt;li{{ item.attributes.addClass(zebra) }}"&gt;{{ item.content }}&lt;/li&gt;
  {% endfor %}
&lt;/ul&gt;


If you still get an WSOD enable error reporting to see in which file and line number the error occurs.