[Serna] Compiling non-standard attributes with DITA open toolkit

Hi,
I have a simple structure with some ditamaps and some topics, which I try to
compile with 1.4.1 DITA open toolkit. The compilation fails because original
schemas don't define attributes xmlns:xsi and xsi:noNamespaceSchemaLocation,
which Serna adds to each created DITA file. Also the Publish command from Serna
produces warnings for each such file.

How can I configure Serna to not add not standard attributes to these files, or
change relevant schemas in the toolkit directory so these attributes are
recognized and don't produce any errors or warnings?

Many thanks for your help in advance.
Kind Regards
Grzegorz Junka