Files not available for selection

When I click upload, all files in the browser window are blanked out and not available for selecting. I have the latest version installed. I've checked to make sure that the allowed extensions are properly defined. I've tried disabling and enabling file uploads (though these two are unrelated).

It was working last week, so I'm wondering if something I've changed on the site in the meantime has disabled FileField. Perhaps there's a conflict or a setting I'm not aware of?