How to "Insert text from file" using OpenXml SDK 2.0
Using Word 2010 GUI, there is an option to "Insert text from file...", which does exactly that: It insert the text in the main part of a document to the current location in your document.

I would like to do the same using C# and the OpenXml SDK 2.0


    using (var mainDocument = WordprocessingDocument.Open("MainFile.docx", true);
    {
        var mainPart = mainDocument.MainDocumentPart;
        var bookmarkStart = mainPart
                            .Document
                            .Body
                            .Descendants<BookmarkStart>()
                            .SingleOrDefault(b => b.Name == "ExtraContentBookmark");
        var extraContent = GetTextFromFile("ExtraFile.docx");

        bookmarkStart.InsertAfterSelf(extraContent);
    }

I have tried using plain Xml (XElement), using OpenXmlElement (MainDocumentPart.Document.Body.Descendants), and using AltChunk. Every alternative so far has yielded a non-conformant docx-file.

What should the method `GetTextFromFile` look like?
How to "Insert text from file" using OpenXml SDK 2.0
===

Using Word 2010 GUI, there is an option to "Insert text from file...", which does exactly that: It insert the text in the main part of a document to the current location in your document.

I would like to do the same using C# and the OpenXml SDK 2.0


    using (var mainDocument = WordprocessingDocument.Open("MainFile.docx", true);
    {
        var mainPart = mainDocument.MainDocumentPart;
        var bookmarkStart = mainPart
                            .Document
                            .Body
                            .Descendants<BookmarkStart>()
                            .SingleOrDefault(b => b.Name == "ExtraContentBookmark");
        var extraContent = GetTextFromFile("ExtraFile.docx");

        bookmarkStart.InsertAfterSelf(extraContent);
    }

I have tried using plain Xml (XElement), using OpenXmlElement (MainDocumentPart.Document.Body.Descendants), and using AltChunk. Every alternative so far has yielded a non-conformant docx-file.

What should the method `GetTextFromFile` look like?