Can't remove file association to STL files

Windows 11’s Default apps interface lets you replace the handler for a file type, but it does not offer a “no application” choice once an association exists. Resetting defaults restores Microsoft-recommended handlers; it does not clear extensions, and manually deleting UserChoice registry entries is not a supported method. Check CHITUBOX settings and disable registration of STL files. Go to Settings > Apps > Default apps, search for .stl, and assign the program you prefer. If you want no installed handler, uninstall CHITUBOX, restart, and test by double-clicking an STL file. Reinstall it only after confirming its installer does not reclaim the extension. If the association remains after uninstalling, use Settings > Apps > Installed apps to repair or remove the registering application, then restart Explorer. Avoid further registry edits because Windows can recreate protected associations. Windows has no supported UI workflow that guarantees a known extension remains permanently unassociated.