Files are served as html instead of plaintext

Problem/Motivation

This module has been serving the ads.txt and robots.txt files as content type html on my site, which is causing issues.

Steps to reproduce

Set an ads.txt file and check the content type it's being sent as.

Proposed resolution

Hard set the content type to 'text/plain' when serving files

Remaining tasks

none

User interface changes

none

API changes

none

Data model changes

none