Streaming In Memory Word Document using OpenXML SDK w/ASP.NET results in "corrupt" document

I am unable to stream a word document that I create on the fly down to the browser. I am constantly getting a message from Microsoft Word that the document is corrupt.

When I run the code via a Console Application and take ASP.NET out of the picture, the document is generated correctly with no problems. I believe everything centers around writing the file down.

Here is my code:

using (MemoryStream mem = new MemoryStream())
{
    // Create Document
    using (WordprocessingDocument wordDocument = WordprocessingDocument.Create(mem, WordprocessingDocumentType.Document, true))
    {
        // Add a main document part. 
        MainDocumentPart mainPart = wordDocument.AddMainDocumentPart();

        new Document(new Body()).Save(mainPart);

        Body body = mainPart.Document.Body;
        body.Append(new Paragraph(new Run(new Text("Hello World!"))));

        mainPart.Document.Save();
        // Stream it down to the browser

        // THIS IS PROBABLY THE CRUX OF THE MATTER <---
        Response.AppendHeader("Content-Disposition", "attachment;filename=HelloWorld.docx");
        Response.ContentType = "application/vnd.ms-word.document";
        mem.WriteTo(Response.OutputStream);
        Response.End();
    }
}

I have looked at a lot of links – but nothing quite works. I lot of people use MemoryStream.WriteTo and some use BinaryWrite – at this point I'm not sure what the correct way is. Also I've tried the longer content type, i.e. application/vnd.openxmlformats-officedocument.wordprocessingml.document but no luck.

Some screenshots – even if you try to recover you get the same "parts are missing or invalid"

Solution for those who stumbl