I thought I would post this in the thought that it might save someone the hour I have just had, chasing my tail through all sorts of WordPress code, to ultimately realise I was to blame.

In a recent site upgrade to 3.3 the media uploader stopped working, so I put in fresh /wp-admin/ and /wp-includes/ but no joy.  It transpires that I had left a redirect in .htaccess to catch .html files from the original site (before we took over) and these were catching the file ‘plupload.html5.js’ and redirecting on.  I immediately then correct my code to make sure the redirect only acted on files ending in .html and all was happy once more.

I have seen a few forum posts unanswered and if you want to test the above quickly:

• Go here: http://www.webconfs.com/http-header-check.php (opens in a new window)
• For the URL use: http://www.yourdomain.com/wp-admin/load-scripts.php?load=plupload-html5
• You can then see the response, of which 200 would be correct, others may need investigating

If the above returns a 200 and you still have a problem, then best to do as I did at the start, install fresh folders of /wp-admin/ and /wp-includes/, reset to a standard theme and test again.