As someone else asked about this from elsewhere, I thought I’d just add a few more details.
For an example on how to add a template setting to your publish plugin, you could use the Maya plugin as an example. The plugin defines the publish template here:
And the actual template value is assigned in the environment settings here:
Though in the case of the question this wouldn’t want to be configured quite like this, as you would want to get the appropriate setting containing the template for the given file type instead.
The plugin then makes use of the template here: