Javascript: Uploading a file... without a file

If you don't need support for older browsers you can use the FormData Object, which is part of the File API:

var formData = new FormData();
var blob = new Blob(['Lorem ipsum'], { type: 'plain/text' });
formData.append('file', blob,'readme.txt');

var request = new XMLHttpRequest();
request.open('POST', 'http://example.org/upload');
request.send(formData);

File Api is supported by all current browsers (IE10+)

• https://developer.mozilla.org/en-US/docs/Web/API/FormData
• http://caniuse.com/#feat=fileapi

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 + '\r\n'
             // Parameter name is "file" and local filename is "temp.txt"
             + 'Content-Disposition: form-data; name="file";'
             + 'filename="temp.txt"\r\n'
             // Add the file's mime-type
             + 'Content-type: plain/text\r\n\r\n'
             + data + '\r\n'
             + 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.

Just sharing the final result, which works - and has clean way of adding/removing parameters without hardcoding anything.

var boundary = '-----------------------------' +
            Math.floor(Math.random() * Math.pow(10, 8));

    /* Parameters go here */
var params = {
    file: {
        type: 'text/plain',
        filename: Path.utils.basename(currentTab.id),
        content: GET_CONTENT() /* File content goes here */
    },
    action: 'upload',
    overwrite: 'true',
    destination: '/'
};

var content = [];
for(var i in params) {
    content.push('--' + boundary);

    var mimeHeader = 'Content-Disposition: form-data; name="'+i+'"; ';
    if(params[i].filename)
        mimeHeader += 'filename="'+ params[i].filename +'";';
    content.push(mimeHeader);

    if(params[i].type)
        content.push('Content-Type: ' + params[i].type);

    content.push('');
    content.push(params[i].content || params[i]);
};

    /* Use your favorite toolkit here */
    /* it should still work if you can control headers and POST raw data */
Ext.Ajax.request({
    method: 'POST',
    url: 'www.example.com/upload.php',
    jsonData: content.join('\r\n'),
    headers: {
        'Content-Type': 'multipart/form-data; boundary=' + boundary,
        'Content-Length': content.length
    }
});

This was tested to work on all modern browsers, including but not limited to:

• IE6+
• FF 1.5+
• Opera 9+
• Chrome 1.0+
• Safari 3.0+