Here's what I'm using - I've been using this for several months now and haven't had an issue:
public ActionResult ChargeSummaryData(ChargeSummaryRptParams rptParams)
{
var fileDownloadName = "sample.xlsx";
var contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
var package = CreatePivotTable(rptParams);
var fileStream = new MemoryStream();
package.SaveAs(fileStream);
fileStream.Position = 0;
var fsr = new FileStreamResult(fileStream, contentType);
fsr.FileDownloadName = fileDownloadName;
return fsr;
}
One thing I noticed right off the bat is that you don't reset your file stream position back to 0.