Format pliku Excel nie można ustalić, musisz określić silnik ręcznie
#When an excel file is opened for example by MS excel a hidden temporary file is created in the same directory:
~$datasheet.xlsx
#So, when you run the code to read all the files from the folder it gives you the error:
Excel file format cannot be determined, you must specify an engine manually.
#When all files are closed and no hidden temporary files ~$filename.xlsx in the same directory the code works perfectly.
Kiiru