Iframes is no longer needed for uploading files through ajax. I've recently done it by myself. Check out these pages:
Using HTML5 file uploads with AJAX and jQuery
http://dev.w3.org/2006/webapi/FileAPI/#FileReader-interface
Updated the answer and cleaned it up. Use the getSize function to check size or use getType function to check types.
Added progressbar html and css code.
var Upload = function (file) {
    this.file = file;
};
Upload.prototype.getType = function() {
    return this.file.type;
};
Upload.prototype.getSize = function() {
    return this.file.size;
};
Upload.prototype.getName = function() {
    return this.file.name;
};
Upload.prototype.doUpload = function () {
    var that = this;
    var formData = new FormData();
    // add assoc key values, this will be posts values
    formData.append("file", this.file, this.getName());
    formData.append("upload_file", true);
    $.ajax({
        type: "POST",
        url: "script",
        xhr: function () {
            var myXhr = $.ajaxSettings.xhr();
            if (myXhr.upload) {
                myXhr.upload.addEventListener('progress', that.progressHandling, false);
            }
            return myXhr;
        },
        success: function (data) {
            // your callback here
        },
        error: function (error) {
            // handle error
        },
        async: true,
        data: formData,
        cache: false,
        contentType: false,
        processData: false,
        timeout: 60000
    });
};
Upload.prototype.progressHandling = function (event) {
    var percent = 0;
    var position = event.loaded || event.position;
    var total = event.total;
    var progress_bar_id = "#progress-wrp";
    if (event.lengthComputable) {
        percent = Math.ceil(position / total * 100);
    }
    // update progressbars classes so it fits your code
    $(progress_bar_id + " .progress-bar").css("width", +percent + "%");
    $(progress_bar_id + " .status").text(percent + "%");
};
How to use the Upload class
//Change id to your id
$("#ingredient_file").on("change", function (e) {
    var file = $(this)[0].files[0];
    var upload = new Upload(file);
    // maby check size or type here with upload.getSize() and upload.getType()
    // execute upload
    upload.doUpload();
});
Progressbar html code
<div id="progress-wrp">
    <div class="progress-bar"></div>
    <div class="status">0%</div>
</div>
Progressbar css code
#progress-wrp {
  border: 1px solid #0099CC;
  padding: 1px;
  position: relative;
  height: 30px;
  border-radius: 3px;
  margin: 10px;
  text-align: left;
  background: #fff;
  box-shadow: inset 1px 3px 6px rgba(0, 0, 0, 0.12);
}
#progress-wrp .progress-bar {
  height: 100%;
  border-radius: 3px;
  background-color: #f39ac7;
  width: 0;
  box-shadow: inset 1px 1px 10px rgba(0, 0, 0, 0.11);
}
#progress-wrp .status {
  top: 3px;
  left: 50%;
  position: absolute;
  display: inline-block;
  color: #000000;
}