A template is shown in the feed adder if
- it has the "Type Template" type
- it has a record field that has been tagged with one of the types of the currently selected object.
The special Record type can be used to tag a field in such a way that any type of object is applicable.
- A file is shown in an activity's feed adder because it has a field that has been tagged with the Activity type. (The field is called "Activity" but the name is not relevant).
- A comment is shown in the feed adder everywhere, because it has a field with the special Record type
If you have limited what's shown in the feed (see "How can I limit what's shown in the feed by default?") you have also limited the templates shown in the adder to those types but the aforementioned conditions also still apply.