Add placeholder support in bootstrap 4 file fields

Author: Jannik Zschiesche

Resources/views/Form/bootstrap_4_layout.html.twig

@@ -119,7 +119,11 @@
         <{{ element|default('div') }} class="custom-file">
             {%- set type = type|default('file') -%}
             {{- block('form_widget_simple') -}}
-            <label for="{{ form.vars.id }}" class="custom-file-label">Choose File</label>
+            <label for="{{ form.vars.id }}" class="custom-file-label">
+                {%- if attr.placeholder is defined -%}
+                    {{- translation_domain is same as(false) ? attr.placeholder : attr.placeholder|trans({}, translation_domain) -}}
+                {%- endif -%}
+            </label>
         </{{ element|default('div') }}>
     </div>
 {% endblock %}