No Flash Uploader Plugin→
With the release of 2.5 came the Flash Uploader functionality, which unfortunately cause some problems in people’s installation.
Some of the problems might already be fixed and will be shipped with the 2.5.1 release, but to those who simply can’t wait, you can use this plugin to disable the Flash uploader (use the “Download in other formats” option at the very bottom of the page to save the plugin’s php file).
[...] I’ve had issues uploading stuff using WP 2.5 so I finally installed a plugin called No Flash Uploader that seems to do the trick. The new gallery is still a bit frustrating, [...]