Hello Christian,
at the moment there exists no API which allows you to read an abitrary file directly within XSJS. Of course it would be possible to define outbound connection to the same system to be able to do an outbound call to get the file content e.g. by using the REST APIs, but this is not really recommended.
Why do you not store the HTML template content in a table? It would be very easy to write a small "template management app" which could upload/download/change templates via a UI5 app. Within your XSJS it would also be very easy to read the template content by $.hdb (or $.db; or XSDS when you have a CDS background).
Best Regards,
Florian