Can't add image to plonesite if AP is installed

when I add new -> image from plone and this package is installed, the store_image_metadata event breaks and prevents me from uploading an image:

```
def store_image_metadata(obj, event):
resource_id = obj.external_img_id
```
obj has not attribute external_img_id

Search the repository for the store_image_metadata function and its registration as an event handler. Reproduce adding an image with the package installed, then determine how the handler should handle objects without external_img_id. Done means image uploads complete without the event raising an attribute error.

Written by the indexing model from the issue text.