Ajax File Upload working in Chrome but not in Firefox

Using XMLHttpRequest Level 2 I want to upload a File. I am using jQuery so I want to do it using jQuery in preference.

So I wrote the following code (coffescript, but should be easily readable by anyone familiar with javascript)

fileToUpload = event.currentTarget.files[0]

data = new FormData()
data.append("uploadedfile", fileToUpload) 

$.ajax({
    type: 'POST',
    url: url,
    data: data,
    contentType: false,
    processData: false,
    mimeType: 'multipart/form-data',
})

Using that code, on the server side with PHP I do a `

With Firefox 17.0.1 I get nothing. $_FILES is null, With Chromium 22.0 I get the the uploaded file (in a strange format, but at least something)

Array
(
    [uploadedfile] => Array
        (
            [name] => filename.txt
            [type] => application/octet-stream
            [tmp_name] => /tmp/phpWwenhc
            [error] => 0
            [size] => 189
        )

)

Does anyone knows if it is a Firefox bug? Is there a workaround? Am I doing something wrong? I searched for a similar problem, but all suggested answers that I found didn't work with me (were not for Firefox, were not using xhr level 2, etc)

Thanks in advance.

PLEASE: I don't care about IE or older versions. Neither iframes solutions nor plugins. Just XmlHttpRequest Level 2. For those who didn't heard about it take a look at: https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest/FormData/Using_FormData_Objects

Remove the mimeType parameter, the browser will generate the correct mime type with the required boundry.