HTTP Headers for File Downloads

As explained by Alex's link you're probably missing the header Content-Disposition on top of Content-Type.

So something like this:

Content-Disposition: attachment; filename="MyFileName.ext"

Acoording to RFC 2046 (Multipurpose Internet Mail Extensions):

The recommended action for an implementation that receives an
"application/octet-stream" entity is to simply offer to put the data in a file

So I'd go for that one.