What are the expected future uses of the dot?
Another approach would be to just remove the warning and allow either dotted or un-dotted extensions to be used, since it's extremely unlikely anyone will intentionally be wanting to handle plugin files in Geany's plugin dir like `theplugin..so`.
In any case, I'm fine with this PR. I actually paste-binned an almost identical patch to @kugel- the other day on IRC when we were discussing this.