I am generating excel files with openpyxl which are fed to PHPExcel. In generating the files with openpyxl 2.4.0-b1 I discovered that PHPExcel was unable to open them.
I found a related bug with PHPExcel that referenced an associated change in openpyxl 2.4.0-a1: https://github.com/PHPOffice/PHPExcel/issues/842
As a result I reverted back to 2.4.0-a1 and was able to successfully open the file with PHPExcel.
Excel files generated by both versions attached for reference.
I have not tested this against the 2.5.x version so if this is already addressed please disregard.