You have to put a rule below that that will hide all items (or items of rarity normal/magic/rare).
The loot filter works from top to bottom. The first rule it finds that matches is the one applied. If no rule is applied, then the default will be used.
This means that when you create your filter, you have to start by hiding everything you don’t want to see. Just create a rule that hides all normal/magic/rare items (and exalted if you want, just make sure to make a rule above it to show the exalted items you want). Make sure that rule stays at the bottom.
You normally have to also hide the items you do not want to see so the others with the affix you want can show up properly. Or if you do not want to hide items you can simply choose “Recolor” instead of “Show” at the rule of the items you want to see, that way when the items with the affix you want drops it will be shown in the color you chose