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

Use CopyTo instead, there is a bug in WriteTo which makes it fail to write the entire content of the buffer when the target stream does not support writing everything in one go.

As variant for .NET Framework 3.5 and lower. This version of framework haven't method CopyTo in class Stream. Therefore, method WriteTo is replaced by next code:

byte[] arr = documentStream.ToArray();
fileStream.Write(arr, 0, arr.Length);

Example was found by http://blogs.msdn.com/b/mcsuksoldev/archive/2010/04/09/creating-a-new-microsoft-word-document-from-a-template-using-openxml.aspx

I believe your ContentType value is incorrect; that is for Word 97 - 2003 format. Change it to:

application/vnd.openxmlformats-officedocument.wordprocessingml.document

and see if that fixes the problem.