Okay, I have finally found a configuration that works.

I took the value from PHP of $_SERVER['REDIRECT_HANDLER'] which is application/x-httpd-ea-php81. (So unfortunately it seems I will have to change this .htaccess rule every time that the PHP version gets updated...?)

Then I put this into the .htaccess file:

<Files test.txt> 
    AddType application/x-httpd-ea-php81 .txt
</Files>