Accept PNG images as photos

Ghost User
2017-09-20T21:08:07+02:00

It should be possible to upload PNG images as photos

### Current behaviour

Uploading a PNG triggers an error.

### Expected behaviour

Uploading a PNG should be handled just like a JPG. Note that this probably also has implications for the thumbnails.

### Steps to reproduce

1. Create a new photo object in the admin
2. Add a PNG
3. Save the object
4. Observe an error