I ended up setting HTTP status to 500 in my Exception filter for all requests but the upload requests, which I detect with this piece of code:

(context.HttpContext.Request.HttpMethod.EqualsOrdinalIgnoreCase( HttpMethod.POST.ToString() ) &&
			       request.ContentType.StartsWith( "multipart/form-data" ))

That seems now to work well for now in all cases.

Thanks for all the help.

Tadeusz