The file extension .UNKNOWN doesn’t mean that a file is a particular UNKNOWN file type but instead it signifies that the file may or may not be of some common type but as the information is not available, the file type is thus classified as “unknown”.

As it is not a separate file type, there is no proper defined file structure for an .UNKNOWN file type. You can try to simply open the .UNKNOWN file using a text editor, if the file is a simple text format, the contents of the .UNKNOWN file can be read. Sometimes, the files with known extension are specifically marked with .UNKNOWN for privacy purposes.