technically anything can open anything
Sure, and I do have my system set so that gedit tries to open some strange "not-really-but-almost text" files and so that ghex opens anything.
But
users are used to see that some kind of files can be opened with "these programs", other kind with "those programs" and so on; and one has to take into account that "users" are prone to customizations: "no, I want this kind of files be opened with this other program".
it is up to you to decide what the file can open that is why i gave you the fmask/mask info
I think your'e misunderstanding what this thread is about. It's not about what my program can open but about which programs the sytem has configured to open "this file" whose name my program has. As I said above, IIRC, imagine that my program is a file manager: It doesn't open files by itself but needs to know which programs open each file.