Send notification from text file content

I would like to use HA to send a scheduled notification message that is basically free text and usually exceeds the 256 character limit for input_text helpers.
I can drop the intended text into a file (eg. in config/www/text/text_file.txt) but am struggling to extract it from there and use it in the notify service call:

service: notify.mobile_app
  message: {{ text pulled from "config/www/text/text_file.txt" }}

Any help is much appreciated.