You may come across STEP files that are polygonal. One way to know is if the file cannot be opened in TransMagic (or any other viewer). When we opened one such file using Notepad++, we saw near the top the line -
FILE_NAME('tinker', '2024-12-09T15:58:56', (''), (''), 'ImageToStl.com STEP Converter','', '');
To open such a polygonal STEP file, use TransMagic's Open Quickview button, which should be to the right of the Open button on the Home toolbar. See screenshot below.