Fix documentation: Colors of `File` (#1574)

docs/documentation/form/file.html

@@ -669,12 +669,12 @@ variables_keys:
 
     <div class="content">
       <p>
-        You can style the file element by appending one of the <strong>9 color modifiers</strong>:
+        You can style the file element by appending one of the <strong>{{ site.data.colors.derived | size }} color modifiers</strong>:
       </p>
       <ul>
-        {% for color in site.colors %}
+        {% for color in site.data.colors.derived %}
           <li>
-            <code>is-{{ color }}</code>
+            <code>is-{{ color.id }}</code>
           </li>
         {% endfor %}
       </ul>