With the merged storage, do not generate log errors for non-image thumbnail requests. Fixes bug #11410.

docs/changelog/7.x.x.txt

@@ -15,6 +15,7 @@
  - fixed #11400: Active Sessions Deletion
  - fixed #11399: m4a missin
  - fixed #11406: AdSku table mssing karma column
+ - fixed #11410: Too much errors in log after fix #11346
 
 7.8.11
  - fixed #11362: Unable to checkout with ITransact plugin

lib/WebGUI/Storage.pm

@@ -1244,8 +1244,11 @@ The file to retrieve the thumbnail for.
 =cut
 
 sub getThumbnailUrl {
-	my $self = shift;
-	my $filename = shift;
+    my $self     = shift;
+    my $filename = shift;
+    if (! $self->isImage($filename)) {
+        return '';
+    }
 	if (! defined $filename) {
 		$self->session->errorHandler->error("Can't find a thumbnail url without a filename.");
 		return '';