The zip files contain the files for an example custom plugin, behavior or theme. The files for an addon can be zipped and renamed .c3addon to directly test it in the Construct 3 editor, via the Addon Manager.
Custom importer API sample data
The plugin SDK download includes a sample plugin using the Custom Importer API. The following sample file can be drag-and-dropped in to Construct 3 to demonstrate reading a custom format.
While developing addons, be sure to use Developer Mode with a local HTTP server. It makes it much quicker to test and much easier to fix problems, which otherwise can prevent Construct 3 from starting up.