Are there any apps for Mac that have Photos extensions?
Looks like this is coming in El Capitan; it was announced at WWDC. 

Reported on multiple news sites, but an example snippet from here: "...support for third-party editing extensions that will be available from the Mac App Store and accessible within Photos itself. It doesn’t sound as though that will enable external editing in Adobe Photoshop, say, but it will go a long way toward enhancing the editing capabilities of Photos."