Why not just use XMLHttpRequest() with POST?

function beginQuoteFileUnquoteUpload(data)

{

    var xhr = new XMLHttpRequest();

    xhr.open(POST, http://www.mysite.com/myuploadhandler.php, true);

    xhr.setRequestHeader(Content-type, application/x-www-form-urlencoded);

    xhr.onreadystatechange = function ()

    {

        if (xhr.readyState == 4 && xhr.status == 200)

            alert(File uploaded!);

    }

    xhr.send(filedata=+encodeURIComponent(data));

}

The handler script at the server just writes the file data to a file.

EDIT

File upload is still a http post with a different content type. You can use this content type and separate your content with boundaries:

function beginQuoteFileUnquoteUpload(data)

{

    // Define a boundary, I stole this from IE but you can use any string AFAIK

    var boundary = ---------------------------7da24f2e50046;

    var xhr = new XMLHttpRequest();

    var body = '--' + boundary + 'rn'

             // Parameter name is file and local filename is temp.txt

             + 'Content-Disposition: form-data; name=file;'

             + 'filename=temp.txtrn'

             // Add the file's mime-type

             + 'Content-type: plain/textrnrn'

             + data + 'rn'

             + boundary + '--';



    xhr.open(POST, http://www.mysite.com/myuploadhandler.php, true);

    xhr.setRequestHeader(

        Content-type, multipart/form-data; boundary=+boundary



    );

    xhr.onreadystatechange = function ()

    {

        if (xhr.readyState == 4 && xhr.status == 200)

            alert(File uploaded!);

    }

    xhr.send(body);

}

If you want to send additional data, you just separate each section with a boundary and describe the content-disposition and content-type headers for each section. Each header is separated by a newline and the body is separated from the headers by an additional newline. Naturally, uploading binary data in this fashion would be slightly more difficult :-)

Further edit: forgot to mention, make sure whatever boundary string isn't in the text file that you're sending, otherwise it will be treated as a boundary.