Context action works on the buffer, so I'm not sure we would want to make filename available as a normal thing, since the buffer might not be saved to the file when the command is performed, so you would need to provide a use-case for it.
If you have a good use-case and you are going to make the change you could make a pull request to contribute it back.
Look in `callbacks.c/on_context_action1_activate()`