jquery form plugin define charset on multipart

jquery form plugin define charset on multipart

Hello all.
I'm  using the jquery plugin form provided by malsup ( http://malsup.com/jquery/) and I have some problems on the encoding of some special character (e.g. àòì).

I need to set the charset on the ajaxsubmit for a multipart form.
I tried to set the correct contenType on the object passed to the ajaxSubmit function as described below:

ajaxSubmit({dataType:"json",type:"POST",contentType: " multipart/form-data;charset=UTF-8"...})

But sniffing on the chrome network console I don't see the charset option:

Content-Type:
multipart/form-data; boundary=---WebKitFormBoundarynAvxcSMbnRzv7GK3

Is there any way to set the charset?