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?

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