When you open a file, EditPad uses the settings you made in the file type configuration to pick a file type for the file. The settings of that file type are then applied to the file. You can pick a different file type for the active file via the File Type submenu in the Options menu.
Changing the file type makes EditPad apply all the settings of that file type to the active file. It does not change the file’s name or extension to match the new file type’s file mask. Use File|Rename / Move to change the file’s name and extension.
If you change a file’s file type via Options|File Type then EditPad Pro remembers that when you close and reopen the file if you tick “preserve file settings” on the Save Files page in the Preferences. EditPad Lite does not have this option.