Well, turns out the problem is the file extensions. FileMaker plugins for Windows are technical just DLLs with a special relation to FileMaker application. So if you rename the .fmx or .fmx64 file to .dll, you can see the metadata in the info dialog:
So if you want to know which MBS Plugin version, you have, please use this trick to read the version name.
For Linux, you can run the file as an application in command line. Each plugin from us has a little command line app in the .so files, which prints the version number to the terminal.