If you are new to 3D printing and are moving, downloading files or viewing folders STL's, you may find your STL extension has changed or renamed when viewing folders containing STL's. The files will appear as a "Certificate Trust List" file. It took me awhile to determine that if you do not have a 3D CAD program installed on said computer, Windows will change its designation since it doesn't have a program associated with it. This usually happens if your internet computer is not the same computer that you do your 3D generating on. The data or extension hasn't changed. Just the default association. Once you move the file(s) over to your 3d app or slicer program, the .stl extension will be visible.
RC
