I have tried this problem, and I confirm that mtypes works correctly, but it's K-Meleon which doesn't verify the mimetypes.rdf parameters before opening. Open seems to only open the default program
I think it's a bug for Dorian.
Mozilla/5.0 (x11; U; Linux x86_64; fr-FR; rv:38.0) Gecko/20100101 Ubuntu/12.04 K-Meleon/76.0
Mail : alain [dot] aupeix [at] wanadoo [dot] fr
Ubuntu 12.04 - Gramps 3.4.9 - Harbour 3.2.0 - Hwgui 2.20-3 - K-Meleon 76.0 rc