Generates rst raw block for given image filename and format

def get_imageblock(filename, format=''):
    """ Generates rst raw block for given image filename and format"""
    _, extension = os.path.splitext(filename)

    with open(filename, "rb") as image_file:
        encoded_string = base64.b64encode(image_file.read()).decode('utf-8')

    return '\n\n.. raw:: html\n\n\t<img src="data:image/' + (format if format else extension[1:]) + ';base64,' + encoded_string +'">\n'