The file extension is simply a conventional way of distinguishing types of files: .pdf for PDF files, .html for HTML files, etc. You can use whatever extension you want (or no extension at all) to store whatever you like. It has no impact on what the file contains.
So if this file is only relevant to your own application, use whatever name and extension you like. You could use .pdf or .html if you wanted to and it wouldn't change anything (except it would be confusing).