added comment to the test

test/test_mediafile_edge.py

@@ -79,6 +79,10 @@ class EdgeTest(unittest.TestCase):
         self.assertEqual(f.bitrate, 0)
 
     def test_only_magic_bytes_jpeg(self):
+        # Some jpeg files can only be recognized by their magic bytes and as
+        # such aren't recognized by imghdr. Ensure that this still works thanks
+        # to our own follow up mimetype detection based on
+        # https://github.com/file/file/blob/master/magic/Magdir/jpeg#L12
         f = open(os.path.join(_common.RSRC, 'only-magic-bytes.jpg'), 'rb')
         jpg_data = f.read()
         self.assertEqual(