HTML files cannot be uploaded as files. What you could do is create a standard page folder for each of those pages, then in the text file for each page reference the name of the html file, then in the template, read those html files.
/content
test/
test.txt
Test.txt
Title: Test
----
Htmlfile: test.html
/site/templates/test.php
<?php
echo F::read($kirby->root('index'). '/'. $page->htmlfile()); //or wherever you want to put those files