Create thumbnails only when the tab is not focused

www/filepicker/app-filepicker.less

@@ -47,6 +47,11 @@
 
             align-items: center;
 
+            img {
+                max-width: 100px;
+                max-height: 100px;
+            }
+
             .cp-filepicker-content-element-name {
                 overflow: hidden;
                 text-overflow: ellipsis;