What knowledge is needed to read unknown file extensions.

You would need to know the structure of the file, but you might not be able to find it if it's proprietary. If you have a converter available you might be able to convert a file into a file type you can read (in this case, maybe something like MP3 would be easier to work with).

Beyond that, there isn't much you can do, all files are just a series of bytes, there's no magic way to translate any file into a useable form.

This page[^] looks like it has some information that could point you in the right direction.

The root of the problem is this: there is no such thing as "file name extension" in Windows or, I'm almost sure, none of the OS where CLI is presently supported. Such "extensions" was in use in some old OS as VAX VMS and MS-DOS (not quite an OS though), probably still in use somewhere, but this "feature" is rather esoteric, more of "limitation" of file system than "extension".

For Windows, this is nothing but some file naming convention. It cannot be known or unknown. Some name patterns are registered in Windows Registry (and more can be added with software installation, for example), only to tell the Windows Shell what default application to use with such files, as well as some other (optional) usage information. The real information on the file structures and formats is contained in the file contents (this is called "metadata", which may or may not be available). That's it.

What knowledge? Perhaps you just need to know a bit more on the concept of the file, file system, file system objects, I/O programming and serialization; everything else depends on particular applications and application fields.

—SA