STL (StereoLithography) is a CAD/CAM format also used for downloadable object files to be sent to 3-D printers. It is based on the vertex coordinates of triangles, and can be saved in binary or ASCII form.
Windows will mis-identify STL files as Certificate Trust Lists, since those use the same extension.
STL files in ASCII form start with "solid" followed by a space and an optional name.
Binary form files have an 80-character header that is generally ignored, but shouldn't start with "solid" so that the file is not mistaken for the ASCII form.