If you have a file that has an UNKNOWN file extension, it does not have a known file association. Therefore, you cannot open the file by double-clicking it.

If the file is a text file, you may be able to open it using a text editor. If it is saved in a binary format, a text editor will not be able to correctly display the file's contents. Therefore, you may need to try dragging the file to several possible applications to attempt to open it.