If you have an extended attribute with Data Type 'Object' and you want to limit the values that can be chosen, you need to use a GTL template. I couldn't find much information on how to do this so thats why i wrote this short tutorial.
You need to create a GTL template for the list of values. You can do that by clicking on the 'new' button at the end of the input field.
You can then write a normal GTL template but you have to return an ObjectID and a tab in the GTL template like this:
.foreach_item(Attributes,,,,) %ObjectID%\t %Name% \n .next (\n)
In this case, the extended attribute is defined on entity level and the GTL template will return all attributes from the corresponding entity.