Changing the format of a file is not simply a case of specifying a different content type or file extension. The content of a PDF file cannot be interpreted as an Excel file.
You need to specify the correct
for the file type you're trying to export. You'll also need to specify the correct content type and file extension.
Stream stream = rpt.ExportToStream(CrystalDecisions.Shared.ExportFormatType.Excel);
return File(stream, "application/vnd.ms-excel", "eNtsaRegistrationForm.xls"); ExportFormatType Enumeration (CrystalDecisions.Shared) | Microsoft Docs