How do you automate text file upload daily to a Drupal Page?
it's much more simple, use Mailhandler

https://www.drupal.org/project/mailhandler

Mailhandler can connect your inbox and with feed you fetch the messages, so basically you need to run the feed using cron job. 
you will find all of that in the configuration of the feed and mailhandler.

Also you can map the attachment with the field where you keep your text file, I don't think there is need to create any plugins, everything you need in mailhandler &amp; feed.