WordPress 2.5 uses a new Adobe Flash based Image and Media Uploader which supports ability to upload multiple files at one go with upload progress bar. However, a lot of bloggers and users encounter problems when using this new “Add Media” wizard dialog.
In Internet Explorer 7 (IE7), when opening the “Add Media” Uploader dialog, the uploader using Flash technology was absent or not showing, and instead be replaced with the standard “Browse File” input box with script errors, as described in WordPress Trac.
The reason of the issue is due to two script errors that do not compatible with IE browser. Developers of WordPress have came out with solution to fix and patch the issue, which will be updated in WordPress 2.5.1. In the meanwhile, before version 2.5.1 is released, for those who wants to use IE7 to use the new Flash Media Uploader can download the changeset_r7573.zip which contains the following 2 files:
Replace these two files in the location above and overwrote by the new downloaded files will fix the IE7 and WordPress 2.5 Flash based image and media uploader issue.