Corrected the comparison. Changed == to eq and introduced lc.

src/lib/Sympa/Message.pm

@@ -1264,7 +1264,7 @@ sub _merge_msg {
 
     # Check for attchment-part, which should not be changed
     my $cdisposition = $entity->head->mime_attr('Content-Disposition');
-    if ($cdisposition and $cdisposition == 'attachment') {
+    if ($cdisposition and lc($cdisposition) eq 'attachment') {
         $log->syslog('notice', 'Detected part with Content-Disposition. Not changing it!');
         return $entity;
     }