For this Reply I used the Reply button instead of the Quick Reply box.
If you use the Reply button you should get a Paper Clip icon about midway through the options. I've never gotten that to work for me. In both Reply and Quick Reply there is also an image icon that I've also never gotten to work for me.
I typically use the Quick Reply and Advanced Options "Attach Files" below the post as described above. Attach files is also available in the Reply option.