Files app: You have to use "contentThumbnailUrl" for artwork, and fix nits.

ui/file_manager/file_manager/foreground/js/ui/single_file_details.js

Index: ui/file_manager/file_manager/foreground/js/ui/single_file_details.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/single_file_details.js b/ui/file_manager/file_manager/foreground/js/ui/single_file_details.js
index 6a191a82962f9be7d8da695988d0172de115febe..7cfe0710f437da7dbc81d8e4ae889a305cc4fbdc 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/single_file_details.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/single_file_details.js
@@ -135,7 +135,7 @@ SingleFileDetailsPanel.prototype = {
             video.poster = result.data;
           }.bind(this));
         }
-      }else if (thumbnailUrl) {
+      } else if (thumbnailUrl) {
         video.poster = thumbnailUrl;
       }
       this.preview_ = video;
@@ -169,7 +169,7 @@ SingleFileDetailsPanel.prototype = {
           }
           var img = document.createElement('img');
           this.thumbnail_.appendChild(img);
-          img.src = item.externalFileUrl;
+          img.src = item.contentThumbnailUrl;
           this.thumbnail_.classList.toggle('loaded', true);
         }.bind(this));
       }