Since you're using FormData, which is a fairly new technology, I'll show you something with new technologies as well.

First, read the file with a FileReader object:

var fr = new FileReader(), buf, file = $('#fileUpload')[0].files[0];

fr.onload = function(e) {

    buf = new Uint8Array(e.target.result);

};

fr.readAsArrayBuffer(file);

Then you can create a Blob for each splitted part (1e6 bytes long each):

for (var i = 0, blobs = []; i < buf.length; i += 1e6)

    blobs.push(new Blob([buf.subarray(i, i + 1e6)]));

Finally, you can add all your Blobs to your FormData object:

var formData = new FormData();

for (var i = 0; i < blobs.length; i++)

    formData.append(slice + i, blobs[i], file.name + .part + i);

You should be ok. I haven't tested it, though.

I don't know anything about the performance either. You can use fr.readAsBinaryString too, thus making e.target.result a string. This way, you can create the Blobs using a simple substring/slice/substr/whatever, but I fear there could be some problems with Unicode characters and whatnot. Plus, maybe it's slower.

Putting everything in a more coherent snippet:

$('#fileUpload').change(function() {

    // If no file is selected, there's nothing to do

    if (!this.files.length) return;



    var fr = new FileReader(), file = this.files[0];

    fr.onload = function(e) {

        splitAndSendFile(new Uint8Array(e.target.result), file);

    };

    fr.readAsArrayBuffer(file);

};



function splitAndSendFile(dataArray, file) {

    var i = 0, formData, blob;

    for (; i < dataArray.length; i += 1e6) {

        blob = new Blob([dataArray.subarray(i, i + 1e6)]);

        formData = new FormData();

        formData.append(fileUpload, blob, file.name + .part + (i / 1e6));

        $.ajax({

            url: 'script.php',

            type: 'POST',

            data: formData,

            processData: false,

            contentType: false,

            success: function(msg){

                alert(Win:  + msg);

            },

            error: function(bla, msg){

                alert(Fail:  + msg);

            }

        });

    }

}

Note: FormData.append takes a third optional parameter, which should be the name of the file in case of File or Blob values. If not specified, Blobs may get unpredictable random file names.

Probably that parameter isn't standard, and it's not mentioned in the MDN artice, but I used it in the snippet above nonetheless. Anyway, if you know what you're doing, you can have several options to specify the file name. For example, with formData.append(filename, file.name) or sending a custom header in the request.