imghdr.what check explicitly for string path, while `open` happily accepts bytes paths, as seen below:

>>> x
b'\xc2\xba'
>>> imghdr.what(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/tank/libs/cpython/Lib/imghdr.py", line 15, in what
    location = file.tell()
AttributeError: 'bytes' object has no attribute 'tell'
>>> open(x)
<_io.TextIOWrapper name=b'\xc2\xba' mode='r' encoding='UTF-8'>

A reason why this should be supported can be found in this message: http://bugs.python.org/msg191691.
The following patch fixes this. Also, it depends on issue19990 (where test_imghdr.py was added).