Thank you for your reply, Allan. After posting, I realized that it was the same as before, I just hadn't noticed it. For file type, it says Certificate Trust List, I don't know what IIS is? I thought the MS updates caused the change, not Mastercam. Microsoft had just installed about six or seven updates at one time, and I thought that it had caused the problem. Of course you are correct, I need to open STL files from within Mastercam. Thanks again for your time. rbrad

Hi, I am a student using Mastercam Demo HLE. A recent windows update changed the default program for opening STL files to "Certificate Trust List." I know how to reset the default program, but I don't know which program was opening it before. Any help, please! Thank you - rbrad