And the tips for jquery ajax call, is to define jquery with this two options (contentType, processData) set to false :

$.ajax({
            url: $url,
            type: 'POST',
            data: formData,
            processData: false, 
            contentType: false,

Ressources :

• FormData and Using FormData Objects
• Uploading File with jQuery Form Plugin, in jQueryUI Dialog Widget
• Uploading file via Jquery ui dialog and Ajax
• How to submit additional form data
• Ridiculously simple Ajax uploads with FormData