Ok, renamed the PHP file to .bin and reflashed successfully. Now I have a lot of work to do to restore my apps.

This could easily have been even more difficult to recover from if there hadn't been enough power left in the battery for a successful flash, since the recharge cycle wasn't working properly.

This experience leaves me very, very unhappy with Nokia.
(and I'm only fogiving Mediabox because it's beta software, use at own risk)