The new revisions of the AP3915i are shipped with a metal mounting bracket now (see here
The bracket is still attached to the AP in the box. Therefore you have to detach the bracket first.
To detach the bracket press strongly in the direction of the red arrow in the picture below until the small metal nose (blue arrow), that is present on the left and right, jumps out of the groove.
Don't worry, nothing breaks even if you have to press a little harder.