diff -r d8674633df18 Lib/imghdr.py
--- a/Lib/imghdr.py	Sun Jan 19 20:47:01 2014 +0200
+++ b/Lib/imghdr.py	Sun Jan 19 21:08:18 2014 +0200
@@ -8,7 +8,7 @@
 
 def what(file, h=None):
     if h is None:
-        if isinstance(file, str):
+        if isinstance(file, (str, bytes)):
             f = open(file, 'rb')
             h = f.read(32)
         else:
diff -r d8674633df18 Lib/test/test_imghdr.py
--- a/Lib/test/test_imghdr.py	Sun Jan 19 20:47:01 2014 +0200
+++ b/Lib/test/test_imghdr.py	Sun Jan 19 21:08:18 2014 +0200
@@ -1,5 +1,7 @@
 import imghdr
 import unittest
+import os
+import io
 from test.support import findfile
 
 TEST_FILES = (
@@ -45,6 +47,16 @@
                        b'GIF80'):
             self.assertIsNone(imghdr.what(None, header))
 
+    def test_file_argument(self):
+        for filename, expected in TEST_FILES:
+            filename = findfile(filename, subdir="imghdrdata")
+            self.assertEqual(imghdr.what(os.fsencode(filename)),
+                             expected)
+            with open(filename, 'rb') as stream:
+                data = stream.read()
+            with io.BytesIO(data) as stream:
+                self.assertEqual(imghdr.what(stream), expected)
+
 
 if __name__ == '__main__':
     unittest.main()
\ No newline at end of file