Posting a form via AJAX through a hidden iFrame. This is necessary when uploading an image using ajax for IE < 10. It is also useful in other cases, where your post endpoint cannot easily digest JSON, for instance.

upload_via_iframe

// This particular code works with html that looks like this:

<div class="upload_container" id="photo_number_1">
    <div><input name="upload" class="photo_upload_field" type="file" /></div>
    <div><input type="button" class="upload_photo_submit" value="Upload Photo"></div>
    <p class="center loading hidden"><img src="/assets/loading.gif"></p>
    <script>
        // yes, I know, don't actually put this in a script tag here. It's for illustration purposes only
	$('.upload_photo_submit')[0]).click(function(event){ajaxFileUpload(event);});
    </script>
</div>

// This can of course be made generic to apply to many different inputs, but I'm lazy.
// This just goes in your JS files.
var ajaxFileUpload = function(event){
  var p_container = $(event.target).closest('.upload_container');
  var upload_field = $(p_container.find('.photo_upload_field')).first();
  if ((upload_field.val() == '') == true) {
    alert('Please select a file to upload.');
  } else {
    p_container.find(".loading").removeClass('hidden');
    if (isAjaxUploadSupported()) {
      var fData = new FormData();
      fData.append('upload',upload_field[0].files[0]);
      $.ajax({
        url:'/photo/upload',
        enctype: 'multipart/form-data',
        type: 'POST',
        data: fData,
        processData: false,
        contentType: false,
        cache: false,
        success: function (data, status) {
          addClassifiedPhoto(data, p_container);
        },
        error: function (data, status, e) {
          alert(JSON.stringify(data['responseJSON']));
        },
        complete: function() {
          p_container.find(".loading").addClass('hidden');
        }
      })
    } else { // IE fallback
      var iframe = document.createElement("iframe");
      iframe.setAttribute("name", "upload_iframe_myFile");
      iframe.setAttribute("id", "upload_iframe_myFile");
	    iframe.setAttribute("width", "0");
      iframe.setAttribute("height", "0");
      iframe.setAttribute("border", "0");
      iframe.setAttribute("src","javascript:false;");
      iframe.style.display = "none";
   
      var form = document.createElement("form");
      form.setAttribute("target", "upload_iframe_myFile");
      form.setAttribute("action", "/photo/upload/?ctype=text%2Fhtml");
      form.setAttribute("method", "post");
      form.setAttribute("enctype", "multipart/form-data");
      form.setAttribute("encoding", "multipart/form-data");
      form.style.display = "none";
		  form.appendChild(upload_field[0]);
       
		  document.body.appendChild(form);
		  document.body.appendChild(iframe);
		  iframeIdmyFile = document.getElementById("upload_iframe_myFile");
 
		  // Add event...
		  var eventHandlermyFile = function() {
		    if (iframeIdmyFile.detachEvent) {
		       	iframeIdmyFile.detachEvent("onload", eventHandlermyFile);
        } else { 
		      	iframeIdmyFile.removeEventListener("load", eventHandlermyFile, false);
        }
        response = getIframeContentJSON(iframeIdmyFile);
        if (response.success == 'false') {
          alert('Failed to upload');
          p_container.find(".loading").addClass('hidden');
        } else {
          addClassifiedPhoto(response, p_container);
          p_container.find(".loading").addClass('hidden');
        }
      }
 
      if (iframeIdmyFile.addEventListener){ iframeIdmyFile.addEventListener("load", eventHandlermyFile, true);}
      if (iframeIdmyFile.attachEvent) { iframeIdmyFile.attachEvent("onload", eventHandlermyFile);}
		  form.submit();
    }
  }
}

function isAjaxUploadSupported(){
  var input = document.createElement("input");
  input.type = "file";
  return (
         "multiple" in input && typeof File != "undefined" && typeof FormData != "undefined" &&
         typeof (new XMLHttpRequest()).upload != "undefined" );
}

function getIframeContentJSON(iframe){
  //IE may throw an "access is denied" error when attempting to access contentDocument on the iframe in some cases
  try {
    // iframe.contentWindow.document - for IE<7
    var doc = iframe.contentDocument ? iframe.contentDocument : iframe.contentWindow.document, response;
    var innerHTML = doc.body.innerHTML;
    //plain text response may be  wrapped  in <pre> tag
    if (innerHTML.slice(0, 5).toLowerCase()  == "<pre>" && innerHTML.slice(-6).toLowerCase() == "</pre>") {
      innerHTML = doc.body.firstChild.firstChild.nodeValue;
    }
    response =  eval("(" + innerHTML + ")");
  }
  catch(err){
    response = {success: false};
  }
  return response;
}

//IE may throw an "access is denied" error when attempting to access contentDocument on the iframe in some cases

Maybe is when x-frame-options is set to deny?