The answer here was actually fairly simple, if you run your own webserver. If you do not, you'll have to point your host to this post.
It turns out that Apache and IIS don't have the new MS Office file formats (
pptx, etc.) listed on their Mime Types table, so IE doesn't know specifically what they are.
Apparently the new Open XML Document format types are types of zipped files, so IE see them as a
zip, and that is why you are forced to save it. The difference between IE and the others (FF, Chrome and Safari I have tested) is that the makers of the others had the foresight to tell their browsers what to do with these file types - it seems the the Microsoft IE team are lazy as well as inept!
Apach Fix -
/etc/mime.types file, add the following line and save the file.
application/vnd.openxmlformats docx pptx xlsx
Now restart the
httpd service and you are good to go (don't forget to reload your page before you test clicking on the attachment).
For more information click here.
See this Microsoft KB article.
Suggested MIME Types for other MS Office extensions -