It seems there are no real HTTP headers to force the UserAgent to prompt for a filename.

From this Stackoverflow answer, it seems that you can simply add the download attribute to your link pointing to the file:

<a href="./my-data.txt" download>Click here to download your data</a>
[download]

But you should be able to achieve the same with just a Content-Disposition header:

Content-Disposition: attachment; filename="my-data.txt"
[download]

There is no way that you can force the user agent to download the data to a file though.