In general the patch LGTM, but additional tests can be added.

Needed various tests for imghdr.what() arguments:

* First argument is bytes, int or None.
* First argument is a name of non-existent file.
* First argument is text stream.
* First argument is output stream.
* First argument is closed stream.
* First argument is non-seekable stream.
* Second argument is bytearray.
* Second argument is str.
* Second argument is None.

Try to insert test function which handles one of already supported format at the front of imghdr.test.