[apolon13]

Попробуйте так

<form id="ajaxform" enctype="multipart/form-data">
      <input class="form-field"  type="file" name='filename' /><br />
      <input class="submit-button" type="submit" value="загрузить" />
 </form>

$("#ajaxform").submit(function(e){ 
var fd = new FormData();
fd.append('image', $('input[type=file]')[0].files[0]);
$.ajax({ 
type: 'POST', 
url: 'gogogo.php', 
data: fd,
processData: false,
contentType: false,
});