formdata js

var form = $('form')[0]; // You need to use standard javascript object here
var formData = new FormData(form);

or specify exact data for FormData();

var formData = new FormData();
formData.append('section', 'general');
formData.append('action', 'previewImg');
// Attach file
formData.append('image', $('input[type=file]')[0].files[0]); 

Sending form

Ajax request with jquery will looks like this:

$.ajax({
    url: 'Your url here',
    data: formData,
    type: 'POST',
    contentType: false, // NEEDED, DON'T OMIT THIS (requires jQuery 1.6+)
    processData: false, // NEEDED, DON'T OMIT THIS
    // ... Other options like success and etc
});
After this it will send ajax request like you submit regular form
with enctype="multipart/form-data"

Update: This request cannot work without type:"POST" in options since all
files must be sent via POST request.