Generates img_id.

def generate_img_id(profile, ext=None, label=None, tmp=False):
    """
    Generates img_id.
    """
    if ext and not ext.startswith('.'):
        ext = '.' + ext
    if label:
        label = re.sub(r'[^a-z0-9_\-]', '', label, flags=re.I)
        label = re.sub(r'_+', '_', label)
        label = label[:60]
    return '{profile}:{tmp}{dtstr}_{rand}{label}{ext}'.format(
        profile=profile,
        tmp=(dju_settings.DJU_IMG_UPLOAD_TMP_PREFIX if tmp else ''),
        dtstr=datetime_to_dtstr(),
        rand=get_random_string(4, 'abcdefghijklmnopqrstuvwxyz0123456789'),
        label=(('_' + label) if label else ''),
        ext=(ext or ''),
    )