I see what you are trying to do, but I don't think it's possible to use a system variable directly in a URL - the URL interpreter (browser) doesn't know what to do with it.
It is not Windows Explorer that is being used to open the "link" in the email - it's the default browser.
You can send an absolute path link such as C:\Users\TLXV105\Documents\Magic Briefcase\subfolder_name\test_document.doc and this will work, provided that the path exists on the target system.
In your case, I don't see why you need the variable (which is causing the problem), because for the variable to work, the path associated with it must exist on the target system, so it could just as well be an absolute path.
SD