fixup! writer_png_Export seems not generating a good result from cloudooo in LibreOffice 4.3.3.2.

cloudooo/handler/ooo/tests/testOooMimemapper.py

@@ -39,7 +39,7 @@ text_expected_tuple = (
 #    ('jpg', 'JPEG - Joint Photographic Experts Group'),
     ('odt', 'ODF Text Document'),
     ('pdf', 'PDF - Portable Document Format'),
-    ('png', 'PNG - Portable Network Graphic'),
+#    ('png', 'PNG - Portable Network Graphic'),
     ('rtf', 'Rich Text'),
     ('txt', 'Text - Choose Encoding'),
     )