Use checkerboard pattern as the background of Quick View images.

ui/file_manager/file_manager/foreground/elements/files_safe_img_webview_content.css

Index: ui/file_manager/file_manager/foreground/elements/files_safe_img_webview_content.css
diff --git a/ui/file_manager/file_manager/foreground/elements/files_safe_img_webview_content.css b/ui/file_manager/file_manager/foreground/elements/files_safe_img_webview_content.css
index c939cf69e3f9ee9526e2559b9e49f4e2966df356..a1c91e75221937edbf9e4100f1ee763e4cb83f14 100644
--- a/ui/file_manager/file_manager/foreground/elements/files_safe_img_webview_content.css
+++ b/ui/file_manager/file_manager/foreground/elements/files_safe_img_webview_content.css
@@ -3,7 +3,6 @@
  * found in the LICENSE file. */
 
 #content {
-  background-color: transparent;
   bottom: 0;
   left: 0;
   margin: auto;
@@ -15,6 +14,17 @@
   top: 0;
 }
 
+img {
+  background-color: white;
+  background-image:
+    linear-gradient(45deg, #efefef 25%, transparent 25%, transparent 75%,
+        #efefef 75%, #efefef),
+    linear-gradient(45deg, #efefef 25%, transparent 25%, transparent 75%,
+        #efefef 75%, #efefef);
+  background-position: 0 0, 10px 10px;
+  background-size: 21px 21px;
+}
+
 #content[hidden] {
   display: none;
 }