The image import re-encodes valid image formats

The image import is supposed to only force the image to become a JPEG image when it is not a JPEG, JPEG2000 or PNG image, but since !224 (merged), it always does. This results in some images being unnecessarily re-encoded.

This seems to be caused by ImageOps.exif_transpose making the image format be None:

>>> img = Image.open('600x600.jpg')
>>> img.format
'JPEG'
>>> ImageOps.exif_transpose(img).format
>>> ImageOps.exif_transpose(img).format is None
True