Combining multiple text documents into one using OpenXML and XElement

As the title says, I'm trying to combine several words (.docx) into one doc word. Each of these documents is one page. I am using part of the code from this post in this implementation. The problem that I encountered is that only the first document is correctly written, each iteration adds a new document, but the contents of the document are the same as the first.

Here is the code I'm using:

//list that holds the file paths
List<String> fileNames = new List<string>();
fileNames.Add("filePath");
fileNames.Add("filePath");
fileNames.Add("filePath");
fileNames.Add("filePath");
fileNames.Add("filePath");

//get the first document
MemoryStream mainStream = new MemoryStream();
byte[] buffer = File.ReadAllBytes(fileNames[0]);
mainStream.Write(buffer, 0, buffer.Length);

using (WordprocessingDocument mainDocument = WordprocessingDocument.Open(mainStream, true))
{
    //xml for the new document
    XElement newBody = XElement.Parse(mainDocument.MainDocumentPart.Document.Body.OuterXml);
    //iterate through eacah file
    for (int i = 1; i < fileNames.Count; i++)
    {
        //read in the document
        byte[] tempBuffer = File.ReadAllBytes(fileNames[i]);
        WordprocessingDocument tempDocument = WordprocessingDocument.Open(new MemoryStream(tempBuffer), true);
        //new documents XML
        XElement tempBody = XElement.Parse(tempDocument.MainDocumentPart.Document.Body.OuterXml);
        //add the new xml
        newBody.Add(tempBody);
        string str = newBody.ToString();
        //write to the main document and save
        mainDocument.MainDocumentPart.Document.Body = new Body(newBody.ToString());
        mainDocument.MainDocumentPart.Document.Save();
        mainDocument.Package.Flush();
        tempBuffer = null;
    }
    //write entire stream to new file
    FileStream fileStream = new FileStream("xmltest.docx", FileMode.Create);
    mainStream.WriteTo(fileStream);
    //ret = mainStream.ToArray();
    mainStream.Close();
    mainStream.Dispose();
}

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