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Basically, I am finding that whenever my client tries to upload a file with IE9 using the FileField, then the page just gives up. The throbber keeps on throbbing indefinately, and nothing crashes as such beacuse you can still navigate the page and whatnot, but no file gets uploaded.
If you view the page in IE7 or IE8 mode with the dev toolbar, then the file uploads fine. Unsurprisingly all other browsers are fine ;-)