Index: lib/RT/Attachment_Overlay.pm
--- lib/RT/Attachment_Overlay.pm.orig	2009-06-10 20:40:33 +0200
+++ lib/RT/Attachment_Overlay.pm	2009-08-07 16:57:05 +0200
@@ -313,10 +313,12 @@
         eval { return( Encode::decode_utf8($content)) } || return ($content);
     }
 
+    if (!$enc) {
     eval { Encode::from_to($content, 'utf8' => $enc) } if $enc;
     if ($@) {
         $RT::Logger->error("Could not convert attachment from assumed utf8 to '$enc' :".$@);
     }
+    }
     return $content;
 }