Thanks. I'm not sure what you mean by "integration with an application", but actually I think it is the second one. If I put some debug code in my  upload.php file, I can see the content type is multipart, but the $_FILES array is empty when using Internet Explorer 8 & html4.

Has anybody actually got this setup working?

The download dialog on IE 8 is produced by the text/plain content type. We will try to make it possible to have the text/html content type. Even though it's wrong it would then work on IE on the HTML4 runtime.

Thanks Spocke for the very timely reply. It works a treat, but I am still a bit confused why I need to change the code in the HTML4 runtime to add the inputContainer to the form rather than the container. Is this a bug?

Another point I have picked up is that on IE7 I could not get the file open dialog to appear with the HTML 4 runtime. With a bit of css hacking I got a red border to appear round the shim, and realised it was off to the left of where it should be. The reason is my template sets the body css to margin: auto with a fixed width, positioning the body in the middle of the browser. In IE8 and Firefox, the call to plupload.getPos correctly takes this into account, returning the x as 485 pixels. But in IE7 it seems to be unaware that the body is centred, and returns 45 pixels. I found that this difference is reflected by document.body.offsetLeft so I have changed the html4 runtime's shim positioning code in my copy to:

plupload.extend(document.getElementById(uploader.id + '_iframe_container').style, {
    top : browsePos.y + 'px',
    left : (browsePos.x+document.body.offsetLeft) + 'px',
    width : browseSize.w + 'px',
    height : browseSize.h + 'px'
});

Does this make sense?

I must be missing something here, I can't see how it will work if the inputContainer is appended to the container. Assuming you don't override the default behaviour through the settings, line 60 sets container = document.body. The body contains the form, obviously, so if you add the inputContainer to the container (=document.body) it is effectively an orphaned input element. When you submit the form, the image file is not included in the post data. The inputContainer must be added to the form, or an element inside the form, or you can't upload the image in the form submission.
Any thoughts? Thanks for responding to my questions though Spocke, it may sound like I am moaning, but I appreciate being able to bounce these things off the developer whether they turn out to be bugs or issues in my code.

Got it. We had written our own jQuery plugin instead of using QueueWidget, so that it would fit in well with the way the rest of our code library worked. But in our case, we did not specify the container setting in the call to plupload, as it is optional. If you take a look at the HTML4 runtime you will see that if container is not specified, document.body is used for the container, which does not work. You need to specify the container as an element which is within the form.
I'd suggest 2 small changes, first to correct the documentation for container:

And second to force some sort of error rather than using document.body if the container is not supplied.

Thanks again for your help.
John

Workaround is to outcomment the following line in upload.php:

header('Content-type: text/plain; charset=utf-8');