wrong spec for `Gmail.User.attachment/3`?

Dialyzer gives me an error for this code:

~~~elixir
{:ok, attachment} = Gmail.User.attachment("foo@bar", mail.id, attachment_part.body.attachment_id)
~~~

The message is:

~~~
The pattern can never match the type.

Pattern:
{:ok, _attachment}

Type:
atom()
~~~
