Hi All!
Just to close this and perhaps some one is interested.
I have now had several excellent feedbacks from the IBExpert Support forum which solved the problem.
The relevant part has to be like this:

        select rdb$set_context('USER_TRANSACTION', 'bulkload', '1') from rdb$database into :somevar;
        update files f
            set f.ccy_total_claim_amount_net = :TCACCY,
                f.ccy2_total_claim_amount_net = :TCACCY2,
                f.date_filter = :DATEFILTER
        where f.file_id = :FILE_ID;

So the presence of the transaction property in the variable does the trick.

Kind regards
Christian

P.S.: I still think that this is also helpful in a normal block statement within one db.