Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/themes/classy/templates/content-edit/image-widget.html.twig

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+{#
+/**
+ * @file
+ * Theme override for an image field widget.
+ *
+ * Available variables:
+ * - attributes: HTML attributes for the containing element.
+ * - data: Render elements of the image widget.
+ *
+ * @see template_preprocess_image_widget()
+ */
+#}
+<div{{ attributes }}>
+  {% if data.preview %}
+    <div class="image-preview">
+      {{ data.preview }}
+    </div>
+  {% endif %}
+  <div class="image-widget-data">
+    {# Render widget data without the image preview that was output already. #}
+    {{ data|without('preview') }}
+  </div>
+</div>