I was playing around with some of the advanced features of Alfred and noticed that if I type "Email email@example.com" and press enter, it loads up my Gmail, and I'm off to the races!
However, if I selected a file (in Alfred) and chose email, it opens my OS X email client "Mail" and attaches it there.
This bug might be a limitation of what you can do with web clients, but I figured I'd report it just the same.
I'm running the latest version (3.3.1) and running OSX 10.11.6
Hope that helps,