Python word counter for all files in the current directory
I recommend using pathlib and Path objects instead of os, and you should use a context manager when manipulating files (e.g. with open("file.txt", "r") as file: ...). You also have a lot of repeating code when you're checking extensions, and you keep checking the rest of the if statements even if it's matched an earlier one. And the final else: pass does literally nothing so just remove that.

You could also do something about your nested for loops for sheet, row and cell (you'd typically use zip or itertools.product then) but this is kind of readable and nice so I'm not sure it's worth the conversion.