depending on the file format, one may have to set dbus-switchboard as the mime handler in the "defaults" section of the dbus-switchboard settings.

on my system video/avi, video/mpeg, video/x-ms-asf and video/x-msvideo is set to hildon-mp_ui.desktop, meaning that anything with those mime types gets handed to the maemo media player. double-tapping any of those will turn it green and change the handler to hildon-dbus-switchboard.desktop.