Double click in project tree opens file in new instance

Hello,

when a file of a project is closed it can be opend with an double click. That file should then open in the current project instance, not in a new instance. How can I avoid this?

Many thanks and greetings
padFox

What is the file extension? Does PSPad knows such extension? If not, add it into any highlighter.
You can use right click in project tree to see what happen with file.