Importing .tid or .json files containing tiddler(s)
As per Reet Or use the Page Control Buttons > import to select the tid file or json file and import.
Dragging and dropping plugins is not always correct on node/server versions.
The untitled tiddler usually appears if you ctrl-ins or ctrl-v paste on a tiddler. Or say drag and drop a link or text, great for importing URL's as you discovered.
When dropping plugins and tiddly wiki file make sure you see the geen drop here bar at the top. Unfortunately hidden by the new Menu bar plugin.