Previously, ImageSpecFile was a file-like object, but didn't actually
extend any of the file classes. Because of this, some of Django's file-
handling code was duplicated. More importantly, instances didn't always
behave as one might expect (if one were familiar with ImageFields),
particularly when the source image was empty. This could have been
especially confusing in templates. (For example, because
ImageSpecFields whose source images didn't exist were still truthy.)
