You can implement the Loadable interface to create your own custom loadables.
This is an advanced feature, as the Resource class already wraps logic around blob/plain data for usages like JSON, configuration, levels, etc through XHR (Ajax).
However, as long as you implement the facets of a loadable, you can create your own.
Gets the data that was loaded
Returns true if the loadable is loaded
Begins loading the resource and returns a promise to be resolved on completion
Processes the downloaded data. Meant to be overridden.
Sets the data (can be populated from remote request or in-memory data)
Wires engine into loadable to receive game level events