gistfile1.py

def is_really_image(node, value):
    request = get_current_request()
    tmpstore = MongoTmpStore(request)
    image_uid = value['uid']
    fp = tmpstore[image_uid]['fp']
    fp.seek(0)
    try:
        Image.open(fp)
    except IOError:
        raise colander.Invalid(
            node,
            '"%s" is not an image file' % value['filename'])