ERROR: File extension in a different format when downloading gridview to excel spreadsheet

Best of luck. I'm going to drop a code spam here with my excel parser, which is intended to handle entity Lists and parse them to .xlsx files:

using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;
using System;
using System.Collections.Generic;
using System.IO;

namespace MyApp.Utility
{
///

/// Used to create excel documents from entities
///

public static class ExcelUtility
{
// Create a text cell
private static Cell CreateTextCell(int col , UInt32 index, string text)
{
var intFirstLetter = ((col) / 676) + 64;
var intSecondLetter = ((col % 676) / 26) + 64;
var intThirdLetter = (col % 26) + 65;

var firstLetter = (intFirstLetter > 64) ? (char)intFirstLetter : ' ';
var secondLetter = (intSecondLetter > 64) ? (char)intSecondLetter : ' ';
var thirdLetter = (char)intThirdLetter;

return new Cell(new InlineString(new Text(text)))
{
DataType = CellValues.InlineString,
CellReference = string.Concat(firstLetter, secondLetter, thirdLetter).Trim() + index
};
}

///

/// Takes a list of objects T and enumerates their properties and values to an Excel spreadsheet.
///

/// <typeparam name="T">
/// <param name="data">List of objects</param>
/// <returns>Excel document stream.
public static MemoryStream GetExcel<t>(List<t> data)
{
var fields = typeof (T).GetProperties();

UInt32 rowcount = 0;

var sheetData = new SheetData();
var count = 0;
var cells = new List<openxmlelement>();

// Add header to the sheet
var row = new Row {RowIndex = ++rowcount};
foreach(var item in fields)
{
cells.Add(CreateTextCell(count, rowcount, item.Name));
count++;
}
row.Append(cells);
sheetData.AppendChild(row);

// Add content to sheet
foreach (var item in data)
{
count = 0;
cells.Clear();
row = new Row {RowIndex = ++rowcount};
foreach(var thing in fields)
{
var temp = thing.GetValue(item);
cells.Add(temp != null
? CreateTextCell(count, rowcount, temp.ToString())
: CreateTextCell(count, rowcount, ""));
count++;
}
row.Append(cells);
sheetData.AppendChild(row);
}
return CreateDocument(sheetData);
}

private static MemoryStream CreateDocument(SheetData sheetData)
{
// Create the Excel document
var stream = new MemoryStream();
var document = SpreadsheetDocument.Create(stream, SpreadsheetDocumentType.Workbook);
var workbookPart = document.AddWorkbookPart();
var worksheetPart = workbookPart.AddNewPart<worksheetpart>();
worksheetPart.Worksheet = new Worksheet(sheetData);
document.WorkbookPart.Workbook = new Workbook(new OpenXmlElement[]
{
new FileVersion { ApplicationName = "Microsoft Office Excel" },
new Sheets(new Sheet { Name = "Sheet1", SheetId = 1, Id = workbookPart.GetIdOfPart(worksheetPart) })
});
document.WorkbookPart.Workbook.Save();
document.Close();
return stream;
}
}
}