How to set default file opening application on Android 5?

I have Aldiko Premium ebook reader application and it's a nice app for reading epub ebooks. I can't set it to be the default application for reading epub ebooks stored as files on my micro SD card. Every time I open a epub from File Commander, I am asked which app to choose to open the epub. I also have Moon Reader, so that's in the list as well as Google Book and an option to Install UB Reader. When I go into settings for the Aldiko app, there is no button to Set Defaults. So how can I not be prompted to choose the app each time I open and epub. Android 5.0.2, Sony Compact Z1.