Adding Your Own Files to Your Visual Studio Solution
Despite the file extensions you see in the Add Existing Item dialog box, Visual Studio isn't limited to working with specific kinds of files. If you have some file that you want to include in your project, you can add it in Solution Explorer. If you want to be able to edit it in Visual Studio, you just need to associate its file extension with one of Visual Studio's editors.
To do that, go to Tools | Options | Text Editor | File Extension. Once there, type an extension in the Extension text box in the top left-had corner, pick an existing editor from the dropdown list to the right of the text box, and click the Add button just a bit further to the right. Now, when you click on a file with that extension, Visual Studio will open it using that editor.
You can also create a new custom editor for Visual Studio based on the Core Editor, using the Visual Studio SDK (though, in the most recent version of the SDK, you'll have to write your editor in C++ because C# and Visual Basic aren't supported any more).
Posted by Peter Vogel on 08/07/2018