MAGETWO-88232: Images from WYSIWYG (Text, Slider) do not show up on admin preview nor storefront

- Build stabilization

Author: kieuphan2406

dev/tests/acceptance/tests/functional/Magento/FunctionalTest/Cms/Data/CmsPageData.xml

@@ -33,6 +33,7 @@
         <data key="shareable">Yes</data>
         <data key="value">magento.jpg</data>
         <data key="fileName">magento</data>
+        <data key="extension">jpg</data>
         <data key="content">Image content. Yeah.</data>
         <data key="height">1000</data>
     </entity>