"Content-Type:text/plain" forces to download the file
I cannot reproduce this with this script:
<?php header('Content-Type:text/plain; charset=ISO-8859-15'); echo "This is some text";
However, I can reproduce it with this:
<?php header('Content-Type:text/plain; charset=ISO-8859-15'); echo "\x00This is some text";
Make sure that your content actually is plain ASCII text...
For me this works:
header('Content-Type:text/javascript;');