tweak to work around PlexConnect bug

Apparently, if the "key" value for a DirectoryObject ends with ".jpg" (or ".png"), as it often does when passing an image string into the callback function, PlexConnect assumes that the result is an image file and tells the aTV to expect an image. That's why some menus fail to load on PlexConnect but other clients handle them fine. I'll file an issue for PlexConnect to improve that behaviour but in the meantime, this patch will allow MTVNetworks to work around the issue.