$OpenBSD$
--- UI/MailPartViewers/UIxMailPartViewer.m.orig	Thu Aug  1 12:17:08 2013
+++ UI/MailPartViewers/UIxMailPartViewer.m	Thu Aug  1 14:41:07 2013
@@ -266,7 +266,7 @@
   NSMutableString *filename;
   NSString *extension;
 
-  filename = [self filename];
+  filename = [[self filename] mutableCopy];
   if (![filename length])
     [filename appendFormat: @"%@-%@",
 	      [self labelForKey: @"Untitled"],