Why can't I add another allowed file extension to a Drupal 7 image field?
The Image widget internally allows (and silently enforces post Drupal 6) the use of only the png, gif, jpg and/or jpeg extensions. So, while you can remove an extension from a field, eg, I have removed gif from most of my implementations to avoid anything animated, you can't add any to this list.  I would imagine this is because it in itself displays the image after being uploaded and didn't want to break any displays in older IE browsers in the actual form (plus, who knows how image styles would work on them as well :)