Post Attachement to a Task does not return created_by

If to make the POST call with the following URL for adding an attachment to a task:

tasks/{taskGid}/attachments?opt_fields=gid,name,permanent_url,created_at,created_by.gid

the responce will not contain the created_by field.
All other fields listed in opt_fields are present.

Is it a bug or feature?
How to get created_by.gid together with other field values, not making the separate request?