How to associate a .cs file to a particular designer in Visual Studio?

We use Active Reports with Visual Studio 2008 and 2010. A report is a .cs file, but because is has some references and the class inherits from ActiveReport, VIsual Studio shows a different icon an开发者_开发问答d you can open the file using a report designer, from Active Reports.

My problem is that in some machines, the file doesn't open with the report designer, there's no Active Reports context menu, etc, even though the product is correctly installed. The same file opens up correctly in other machines.

My question:

Do you know how this association is made in Visual Studio? Where do you configure it to use an external application (the report designer) to open a C# file and to show certain context manu items?

Edit: When I right-click on the file and choose "open with" the report editor does not show up in the list. My question is about how that association is done behind the scenes, based on the code, without using the right-click - open with.

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Right-click the file in Solution Explorer. Select Open With.

You can change the default editor for the file or configure new editors.

See: Open With Dialog Box
         How to: Change or Add a Default Editor