A variation of B Diddy's post -
Go to Settings > Apps > scroll over to All. Find the 'default' player (Google Play and Movies for video, Photos for, well, photos) and click on it to open it's info page. Look to the bottom of the page for 'Launch By Default'. Click 'Clear Defaults'. This will do what B Diddy suggested, but specifically for each app.
This should, at the very least, force a choice box to display when you open a file to set a preferred app. That said, I've been running my video through MX Player Pro, GoneMAD Music Player for audio and QuickPic for photos on my Nexus 7 since before the KitKat install and never had to reset the preferences.
A slightly more extreme Plan B: You might want to try disabling the offending app (back to Settings > Apps > All > the app involved. Clear 'Launch by Default', Clear Cache, Clear Data, uncheck Notifications (you're okay with the warning that pops up), Force Stop, Disable (you're okay with it going back to factory install). This will place the app in the Disabled list here - always available for re-enabling (no such thing as Delete for built-in apps) and should remove the app from consideration by Android entirely.
If none of this is working for you, there might be a bug in your install?