Hi mmix,
this is actually an interesting question and while trying to dig a bit into it I found out that you can immediately activate the file preview for .lic files when you change the extension attribute of their entries in the fileinfo table in the database to txt (and restart the server afterwards).
While this doesn’t help you for newly posted .lic files, you could at least make the existing ones work now (if this is relevant, not sure if the preview is only relevant for newly uploaded files).
I’ve asked the developers if it’s possible to generate the preview based on the mimetype (which would help us out in this situation) and will update you as soon as I hear from them.