Saving to disk opens the target folder after saving. Which is done by passing the folder path to windows. Since you have a test.bat file and test folder, when calibre asks windows to open the folder it executes the bat file instead, which presumably happens because microsoft thought it a good idea to execute files with .exe/.bat and .cmd extensions even when the full filename with extension is not supplied. Given that microsoft also thought having magic filenames in their filesystems sunch as NUL and COM was a good idea, I'm not surprised.
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