I am getting a warning message that says "openpyxl/reader/worksheet.py:320: UserWarning: Unknown extension is not supported and will be removed"
I've traced the extension value to be 64002731-A6B0-56B0-2670-7721B7C09600.
It shows up when I open a file that was saved in Excel for Mac 2011. My version is 14.6.8.
Since it's a warning it doesn't halt execution but it would be nice not to receive it. I don't understand enough about what these do to want to touch the code myself.