jQuery Ajax File Upload

File upload is not possible through AJAX.
You can upload file, without refreshing page by using IFrame.
You can check further details here.

UPDATE

With XHR2, File upload through AJAX is supported. E.g. through FormData object, but unfortunately it is not supported by all/old browsers.

FormData support starts from following desktop browsers versions.

• IE 10+
• Firefox 4.0+
• Chrome 7+
• Safari 5+
• Opera 12+

For more detail, see MDN link.

Simple Upload Form

<script>

   //form Submit
   $("form").submit(function(evt){   

      evt.preventDefault();
      var formData = new FormData($(this)[0]);

      $.ajax({
          url: 'fileUpload',
          type: 'POST',
          data: formData,
          async: false,
          cache: false,
          contentType: false,
          enctype: 'multipart/form-data',
          processData: false,
          success: function (response) {
         
             alert(response);
          }
       });

       return false;

    });

</script>

<!--Upload Form-->
<form>
  <table>
    <tr>
      <td colspan="2">File Upload</td>
    </tr>
    <tr>
      <th>Select File </th>
      <td><input id="csv" name="csv" type="file" /></td>
    </tr>
    <tr>
      <td colspan="2">
        <input type="submit" value="submit"/> 
      </td>
    </tr>
  </table>
</form>

Ajax post and upload file is possible. I'm using jQuery $.ajax function to load my files. I tried to use the XHR object but could not get results on the server side with PHP.

var formData = new FormData();
formData.append('file', $('#file')[0].files[0]);

$.ajax({
       url : 'upload.php',
       type : 'POST',
       data : formData,
       processData: false,  // tell jQuery not to process the data
       contentType: false,  // tell jQuery not to set contentType
       success : function(data) {
           console.log(data);
           alert(data);
       }
});

As you can see, you must create a FormData object, empty or from (serialized? - $('#yourForm').serialize()) existing form, and then attach the input file.

Here is more information: - How to upload a file using jQuery.ajax and FormData - Uploading files via jQuery, object FormData is provided and no file name, GET request

For the PHP process you can use something like this:

//print_r($_FILES);
$fileName = $_FILES['file']['name'];
$fileType = $_FILES['file']['type'];
$fileError = $_FILES['file']['error'];
$fileContent = file_get_contents($_FILES['file']['tmp_name']);

if($fileError == UPLOAD_ERR_OK){
   //Processes your file here
}else{
   switch($fileError){
     case UPLOAD_ERR_INI_SIZE:   
          $message = 'Error al intentar subir un archivo que excede el tamaño permitido.';
          break;
     case UPLOAD_ERR_FORM_SIZE:  
          $message = 'Error al intentar subir un archivo que excede el tamaño permitido.';
          break;
     case UPLOAD_ERR_PARTIAL:    
          $message = 'Error: no terminó la acción de subir el archivo.';
          break;
     case UPLOAD_ERR_NO_FILE:    
          $message = 'Error: ningún archivo fue subido.';
          break;
     case UPLOAD_ERR_NO_TMP_DIR: 
          $message = 'Error: servidor no configurado para carga de archivos.';
          break;
     case UPLOAD_ERR_CANT_WRITE: 
          $message= 'Error: posible falla al grabar el archivo.';
          break;
     case  UPLOAD_ERR_EXTENSION: 
          $message = 'Error: carga de archivo no completada.';
          break;
     default: $message = 'Error: carga de archivo no completada.';
              break;
    }
      echo json_encode(array(
               'error' => true,
               'message' => $message
            ));
}

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;
}