You are now a developer, no more a user. So as a user, you as-k for a new window.
As a developer, you may have to create a new window (to put the file contents), but you also have to write the code to load the file contents and place it into the correct Control in the window.
I think you need to look into File Type Groups, here you will detail your application and tell you built application to be linked to your file type (extension etc).
Open your apps source code in Xojo
Select Insert File Type Group from the Insert menu
In the Type enter Editor. Put your extension in. Provide an icon for the file type. For Mac you need to set the check box for ‘file type is unique to this app’ and put in a UTI.
This will tell Finder on Mac and Explorer on Windows that when your unique type is double clicked to open in your application. Is that what you are asking about?