Hard to tell from the photo but possibly the oval washer for the mounting bolt may be limiting the upward adjustment due to the braze-on mounts' shape. Try doing a loose mount without the washer and see if the derailleur can move up any further. If so, try grinding a little of the backside of the washer off so it can move up until the bolt can hit the top of the braze-on slot. If no luck, try the already mentioned, filing the slot a little and there is also the option of drilling out the rivets for the braze-on mount and install a band-on adapter for your braze-on derailleur. A few examples here: front derailleur clamp adapter - Search