The JavaBeans Activation Framework is a library for abstracting the operating system specific bindings/mappings between file types and applications that deal with them. This can be based on MIME types and/or file extensions. It’s really nothing more than that.
Open up Windows explorer and right click on a file. Various actions may be there (eg Open, Edit, Play). What actions are associated with the file is associated with the file extension on Windows. JAF is a portable version of that. It’s mainly intended for desktop applications to be able to interact with whatever programs can be used with files on that computer.