This is most likely using Xls file format. Import of this format type is currently not supported. You can vote for the public item, and subscribe to track it for eventual status changes:
SpreadProcessing: Support for import and export of .XLS documents
RadSpreadsheet operates with a rich document model that is completely decoupled from UI. The documentation of the model can be found in the RadSpreadProcessing section of the documentation for Telerik Document Processing here
Supported file formats are Xlsx, Pdf, Csv, Txt.
If this is not the case, could you share the document you are using?
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