Your mishandled baggage file reference will be located in the PIR - Passenger Irregularity Report, which would be created by the staff member who registered your claim. In the PIR you will have a three letter code followed by the airline code and 5 digits, e.g. DOHQR12345.
For other enquiries contact your local offices, contact details can be found in the following link https://www.qatarairways.com/en/worldwide-offices.html