'If the L{FileBodyProducer} is constructed with a file-like object with
both C{seek} and C{tell} methods, its C{length} attribute is set to the
size of the file as determined by those methods.'

def test_knownLength(self):

    inputBytes = 'here    are    some    bytes'
    inputFile = BytesIO(inputBytes)
    inputFile.seek(5)
    producer = FileBodyProducer(inputFile)
    self.assertEqual((len(inputBytes) - 5), producer.length)
    self.assertEqual(inputFile.tell(), 5)
