Your STP or STL file is a downloaded file.
If a text file has opened in a tab in your browser, please go back to the STP/STL icon and right click, "Save Link As..." and save the file to your computer. This will save the file to your computer and bypass your personal download settings in your browser.
The STP/STL file cannot be viewed in your internet browser.
Please go to your download folder and open your STP or STL file with your CAD software.
On some older computers an STL will try and open as a Certificate Trust List file. To bypass this setting right click the file and "Open with..." your prefered CAD program.
You can also open your CAD program and import the STL or STP file. Some cad programs require an stp file to be a step file. Just change the file extension from STP to STEP.
If you do not have a file viewer or CAD program compatible with your STP or STL file you can find many free online.
Free software for viewing STP files on the computer:
Free online and desktop software for viewing STL files on the computer: