GIF87a is no longer recognized in 2.5.0+

When python-xmp-toolkit is automatically rebuilt against latest exempi, its tests fail. Unfortunately, the tests are not particularly verbose, but at least this result is clear:

FAIL: test_formats (test.test_exempi.TestExempi)
Verify that check_file_format function works as expected.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builddir/build/BUILD/python-xmp-toolkit-2.0.1/test/test_exempi.py", line 440, in test_formats
    self.assertEqual(actual_format, expected_format)
AssertionError: 538976288 != 1195984416

In this case, 1195984416 = 0x47494620 = XMP_FT_GIF, and 538976288 = 0x20202020 = XMP_FT_UNKNOWN. The code is very simple, it just loops over a bunch of files of varying types and calls xmp_files_check_file_format. With 2.4.5 this worked, but with 2.5.0 or 2.5.1 it fails.