I'm trying to submit a form using JQuery. My problem comes from the fact that the front end (html+js/jquery) and the back-end is not on the same site, but it does support JSONP.
The form contains a file input field, so I would be submitting Multi-part form data. How would you resolve this?
If all you want to do is submit the form an go to the external site (ie as if you pressed a submit button on a tranditional web form), you can just trigger the form's submit method using Javascript; it doesn't matter where the form posts to.
However, if you want to post cross-domain using an AJAX-type method, you'll have a harder time of it. The answer lies in using JSONP rather than JSON in your JQuery AJAX requests.
See the JQuery Ajax documentation for more details.
EDIT: Do not try this, it will not work for cross domain posts. My fault for not reading the question carefully enough.
Does it have to be a form submit? If not, you could simply do a jQuery ajax call that posts json to it similar to this:
your form action would point to the site controlling the post.
then you can call
$('#theForm').submit();