Extensions are kinda lame, aren't they? You can never tell what stuff is.
I suggest using "file" to get hints as to what kind of file it is. e.g., "file op.me" would PROBABLY return something like:
op.me: ASCII troff or preprocessor input text
To save you some time, the best way to view it would be "groffer --ps-viewer ggv op.me"
The --ps-viewer part isn't really necessary, but the default viewer would probably be gs, and I doubt you'd enjoy its interface very much.