If you Import a docx word file into Confluence the UI will show you an error saying that the imported document is "not a valid binary Word 97-2003 document" instead of saying that this is not a valid Word 97-2010 document.

Steps to reproduce:

1. Have a Confluence on version 5.2.5.
2. Use the attached blank document to import into a Confluence page.
3. You will get the following error:
   

Note

If you try to import a docx file from 2013 on Confluence 5.5.3 you will no longer get this error, however it might not import correctly because Confluence is still not fully compatible with MS OFFICE 2013 as you can see in CONF-29342.