.UNKNOWN File Extension

Unknown Windows file that does not have a file association; may have an ".unknown" extension to serve as a placeholder for the file's correct extension.

If you have a file with either no extension or an ".unknown" extension, you can try to view the contents of the file by opening the file with a text editor. However, if the file is saved in a binary format, a text editor will not display the file's contents correctly. If you believe you know what the file type is, you may try changing the extension to the appropriate suffix and then attempt to open the file.