STL subtitle file not recognized after installing STL plugin

Hello, I posted this question in this thread    Stl file in Trados file type not recognised  

After I consider better to start a new one.

I have stl files from a client. I installed both plugin for STL and Studio Subtitling. I restarted Studio after (three times) and the formats appear on the list of type of files. But Studio asks for a translatable file.

Thanks for your help. 

Studio 2021, btw.