Have you restarted the server after making the DoJo 1.5 changes ? This is important so that any server level caches are cleared and the server sends the new files down to the client.
Also, have you made sure your running the latest version of Adobe Flash in your browser. The file upload control is flash based. I have seen an issue in IE where an older version of Flash was being used and the SWFObject is not available in older versions of flash, resulting in the problem of the button not becoming active.