You don’t use a file extension to open a file. The operating system uses a default application to open each file extension type. Without knowing the contents of the file, it could be anything, audio, video, text database or any of a hundred other formats. Where did you get the file from and why is there no extension? What is the size of the file ? You could name a text file with a .MOV extension and a video play will not know what to do with it because the file itself is not in a compatible format.
If this is a common issue for you then here is a link that may help.
It explains step by step how to validate whats inside a file.