You must include the xlsx file extension.
The legacy (closed) binary format of office files is actually the older Xls file type.
But starting with office 2007, Microsoft switched to using the open XML format for ALL of its office documents (well, ok, outlook pst, and MS Access are exceptions). Try renaming the xlsx document as a zip file if you have any office documents (word, power-point, excel, etc.). You discover that you may now open the zip archive and see, examine, and even edit the XML contained therein.
Therefore, more recent office file formats are actually XML markups, and you can edit them!
In any case, the library you're using unquestionably takes use of the fact that office files are now XML-formatted rather than being stored in a proprietary Microsoft binary format. As a result, these files can now be modified on ANY computer that can handle zip files and text files.
As a result, Excel is alerted to the fact that you are naming the document with a legacy binary file extension (Xls) while opening a plain text XML file. Therefore, you must add the open office XML format extension to that document (xlsx).