def is_textfile(filename, max_unprintable=0.05):
    bytes = open(filename).read(512)
    textchars = ''.join(map(chr, [7,8,9,10,12,13,27] + range(0x20, 0x100)))
    unprintable = bytes.translate(string.maketrans('',''), textchars)
    return float(len(unprintable)) / len(bytes) < max_unprintable