Remove temporary file assignment after validation

This can cause problems when the relation is set to a HttpObject and validation fails, during the update, the relation is now set to something invalid for rendering. So we should unset it

src/Database/Relations/AttachOne.php

@@ -79,6 +79,10 @@ public function setSimpleValue($value)
 
         // The relation is set here to satisfy validation
         $this->parent->setRelation($this->relationName, $value);
+
+        $this->parent->bindEventOnce('model.afterValidate', function() {
+            $this->parent->unsetRelation($this->relationName);
+        });
     }
 
     /**