migrate file and image widgets

packages/netlify-cms-widget-file/src/FilePreview.js

@@ -0,0 +1,16 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { WidgetPreviewContainer } from 'netlify-cms-ui-default';
+
+const FilePreview = ({ value, getAsset }) => (
+  <WidgetPreviewContainer>
+    { value ? <a href={getAsset(value)}>{ value }</a> : null}
+  </WidgetPreviewContainer>
+);
+
+FilePreview.propTypes = {
+  getAsset: PropTypes.func.isRequired,
+  value: PropTypes.node,
+};
+
+export default FilePreview;