[mythtv] How does HandleMedia / RegisterMediaHandler work?

[Matthew Wire]

When MythTV starts up with a DVD-video in the drive it is handled by
MythGallery instead of MythDVD.  Both are registered as mediahandlers.

I thought I'd try and fix this but can't work out how the media handler
code works.  The plugin obviously has to declare a handlemedia function
but I assumed it should call a registermediahandler function as well.
However, I can find no reference to this in mythgallery. So:

How does a plugin register a media handler?
How does a plugin register it's handler for specific types only?

Thankyou,
Matthew