Well, if you can attach an image or video directly to a post with the FT app, then you are able to do something in the app that you can't do in the regular FT interface.

Most users in most forums are not able to add an attachment directly to a post. Users must upload the image/video to a separate hosting service and then link (inline for images or as a link for video) to it in a post.

Perhaps it's a permissions issue, then? Though I would have assumed that you, as an admin, have the ability to add an attachment to any post anywhere, and as such it should have worked for you in your test post above...