For the time being the Spreadsheet parser is unable to read files which tags are not in the default namespace. Try to open the attached file in this demo project: https://demos.telerik.com/kendo-ui/spreadsheet/index.
You won't be able to load the file. If you open the file in Excel, save the file ("Ctrl+S") and try to load it in the Spreadsheet again, there will be no issues with it. The difference in file's structure before and after being saved in Excel could be seen in the attached screenshot.
The code in red is the structure of the file before saving it and the one in green is after the "save" operation. The difference between the files is that the structure of the one saved in Excel inserts the tags in the default namespace while the original document uses the "x:" namespace.
It will be very useful if we can load files with defined namespaces in their structure.