When someone downloads a document, the server tells the
browser what type of file it is. The browser then picks the
appropriate helper application, or displays it himself. If
the server doesn't know the file type, it tells the browser
that the file is "text/plain", or just plain text. You will
have to ask your server admin to configure this particular
file with the MIME type you want. 

"Forcing" a download is not what you are supposed to do.
After all, what is more convenient than having the proper
application started when I download a particular file?
Browsing through a download directory can be quite a pain.
And most browsers allow the user to download to disk if they
want to. 

If the file must be saved to disk, as there is absolutely NO
other way to handle it, the MIME type should be
"application/octet-stream".