thank you for your reply. The problem is, that I use the very same form twice in different modes. Initially I use to form to upload images with metadata. As an image can be tagged with various keywords, the keywords are sent as an array.
When I use the form to edit the metadata of an already uploaded image, I send the form data as usual POST data. Hence the keywords are sent like: like keyword_id=142&keyword_id=988....etc. and handled on the server using a PHP script. When I use the form to upload a new image asynchronously, I have to send the data as described in the previous post and I would appreciate if I could handle the request with the very same PHP script without having to fiddle around too much.
Currently I upload the image synchronously to overcome the problem but it would be nice, if an array could be serialized the traditional way using e.data in the upload event.