set vim filetype for files with no extension, only

How do I set up filetype and/or syntax for files that have no extension in vim?

Note

This is a duplicate of vim default syntax for files with no extension. I'm asking again because no one has answered it properly in my view.

I don't want to know how to set the default syntax for unrecognized file types, I want to know how to set it only for extensionless files.