New post form encoding

I have a custom plugin that uses a plugin defined meta box added onto the new posts page to accept and upload media files.

This plugin has worked without any problem on a multi-site network since initial setup back at 3.0.

I’d fallen behind on upgrades so I just jumped from 3.0.1 to 3.0.5 and then to 3.1 and the unloader stopped working. When I investigated I found that the $_FILES array was empty.

I’m able to restore functionality to the site forcing the post form to multipart encoding. I’m doing so using a variation of this JavaScript.

I’m not wild about fixing this using JavaScript. Can someone point me to the change in core between 3.0.1 and 3.0.5 that caused this form encoding problem to arise? Whatever the problem 3.1 did not fix it.