Update UIxMailPartViewer.m

UI/MailPartViewers/UIxMailPartViewer.m

@@ -287,7 +287,7 @@
   if ([filename length])
     // We replace any slash by a dash since Apache won't allow encoded slashes by default.
     // See http://httpd.apache.org/docs/2.2/mod/core.html#allowencodedslashes
-    filename = [[filename stringByReplacingString: @"/" withString: @"-"] mutableCopy];
+    filename = (NSMutableString *)[filename stringByReplacingString: @"/" withString: @"-"];
   else
     [filename appendFormat: @"%@-%@",
 	      [self labelForKey: @"Untitled"],