If the extension is an unknown format, BSplayer will not appear as an Open With option, you should register the extension to BSplayer.
The simplest way to do this is to open BSplayer, open the Preferences window (CTRL+P), go to the "File types" section, and in the white box insert the unknown extension. Then click on the "Add extension" button, look in the list for the added extension and check it. Maybe select the "Register extensions on player start", restart BSplayer
