A file extension can be used, beyond itsu2022 normal purposes, which are to associate the file with an application. For example, one could write an application using arbitrary file extensions to categorize files according to a pre-designed organization. Another option, would be to pra non-standard feature, on the front end of an application association (modify the application to do u201csomething differentu201d, when opened with a file contain property u201cxu201du2022 ), another off the top of my head, would be to embed meaning in a tree of extensions, sorted according to a pre-defined method, such as as a security protocol, using distributed file data (based on extension associations, coupled with a file name sorting algorithmu2026). The question as written suggest to me how the file extension might be used in interesting ways, outside of the scope of its actual initial purpose, which was to prthe OS, information as to the u201ctypeu201d of file, thus the word extension and type in some circles are synonymous.