How to use FormData for AJAX file upload?

This is my HTML which I'm generating dynamically using drag and drop functionality.

<form method="POST" id="contact" name="13" class="form-horizontal wpc_contact" novalidate="novalidate" enctype="multipart/form-data">
<fieldset>
    <div id="legend" class="">
        <legend class="">file demoe 1</legend>
        <div id="alert-message" class="alert hidden"></div>
    </div>

    <div class="control-group">
        <!-- Text input-->
        <label class="control-label" for="input01">Text input</label>
        <div class="controls">
            <input type="text" placeholder="placeholder" class="input-xlarge" name="name">
            <p class="help-block" style="display:none;">text_input</p>
        </div>
        <div class="control-group">  </div>
        <label class="control-label">File Button</label>

        <!-- File Upload --> 
        <div class="controls">
            <input class="input-file" id="fileInput" type="file" name="file">
        </div>
    </div>
    <div class="control-group">    

        <!-- Button --> 
        <div class="controls">
            <button class="btn btn-success">Button</button>
        </div>
    </div>
</fieldset>
</form> 

This is my JavaScript code:

<script>
    $('.wpc_contact').submit(function(event){
        var formname = $('.wpc_contact').attr('name');
        var form = $('.wpc_contact').serialize();               
        var FormData = new FormData($(form)[1]);

        $.ajax({
            url : '<?php echo plugins_url(); ?>'+'/wpc-contact-form/resources/js/tinymce.php',
            data : {form:form,formname:formname,ipadd:ipadd,FormData:FormData},
            type : 'POST',
            processData: false,
            contentType: false,
            success : function(data){
            alert(data); 
            }
        });
   }

For correct form data usage you need to do 2 steps.

Preparations

You can give your whole form to FormData() for processing

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.

Note: contentType: false only available from jQuery 1.6 onwards

I can't add a comment above as I do not have enough reputation, but the above answer was nearly perfect for me, except I had to add

type: "POST"

to the .ajax call. I was scratching my head for a few minutes trying to figure out what I had done wrong, that's all it needed and works a treat. So this is the whole snippet:

Full credit to the answer above me, this is just a small tweak to that. This is just in case anyone else gets stuck and can't see the obvious.

  $.ajax({
    url: 'Your url here',
    data: formData,
    type: "POST", //ADDED THIS LINE
    // THIS MUST BE DONE FOR FILE UPLOADING
    contentType: false,
    processData: false,
    // ... Other options like success and etc
})

<form id="upload_form" enctype="multipart/form-data">

jQuery with CodeIgniter file upload:

var formData = new FormData($('#upload_form')[0]);

formData.append('tax_file', $('input[type=file]')[0].files[0]);

$.ajax({
    type: "POST",
    url: base_url + "member/upload/",
    data: formData,
    //use contentType, processData for sure.
    contentType: false,
    processData: false,
    beforeSend: function() {
        $('.modal .ajax_data').prepend('<img src="' +
            base_url +
            '"asset/images/ajax-loader.gif" />');
        //$(".modal .ajax_data").html("<pre>Hold on...</pre>");
        $(".modal").modal("show");
    },
    success: function(msg) {
        $(".modal .ajax_data").html("<pre>" + msg +
            "</pre>");
        $('#close').hide();
    },
    error: function() {
        $(".modal .ajax_data").html(
            "<pre>Sorry! Couldn't process your request.</pre>"
        ); // 
        $('#done').hide();
    }
});

you can use.

var form = $('form')[0]; 
var formData = new FormData(form);     
formData.append('tax_file', $('input[type=file]')[0].files[0]);

or

var formData = new FormData($('#upload_form')[0]);
formData.append('tax_file', $('input[type=file]')[0].files[0]); 

Both will work.