As Mark Hayes said, you cannot use the handy "Quick Reply" area.
You have to click "Reply" on a specific post .. then in the resulting window scroll down a little and look for the tiny little File Manager link. Works great (usually) once you find it.

It is always available in an new post, but in a reply there is a certain click path needed to access it.


Make your sound your own!
.. I do not work here, but the benefits are still awesome