How can I open a file from the finder in my OS X Unity App

I’m developing a cross platform application able to open and edit its own kind of files.
On Windows, it was quite easy to open files with my app using the arguments of command line.
On IOS, well it finally worked editing the Info.plist and the in Xcode.
On OS X, well I don’t know how to do it. I already modified the Info.plist file inside the packet of my app so OS X know my App is able to open my file … but I can’t access to nor to AppDelegate.m to handle the file because Unity already compiled the whole thing. Moreover the AppControler file inside the Unity packet seems to be exclusively used for IOS building.
Would you see a way to do it ?
Thank you for your answers.

I finally created a second app with XCode which is able to read my kind of file. This app just write in a text file the path of the file to open and then launch my main Unity app which read the text file and open my file correctly… Yay.
A good think to implements in Unity to create OSX apps would be the generation of an XCode project to access some native functionalities.

Using only relative paths from the .app seems to work fine for me to access files on os x.