Hi Norbert
i had to do that in the past (to clean dummy data from an idempeire db) and I used this SQL script, you just need to finde the master table id (bp, invoices and so on) XXXXXX and the record id (us the record or the master table (owner of the attacherment) for example c_invoice_id and play with the values of your environment ..
delete from ad_attachment where ad_table_id = XXXXXX and record_id = YYYYY ;
Hope that helps you.