title: jQuery-File-Upload inconsistently gives file data
body: <p><a href="https://github.com/blueimp/jQuery-File-Upload" rel="nofollow">jQuery-File-Upload</a></p>
  
  <p>Upload script:</p>
  
  <pre class="lang-js prettyprint-override"><code>$('#fileupload').fileupload({
      url: 'api/combox_upload.php',
      type: 'POST',
      dataType: 'json',
      dropZone: $dropZone,
      singleFileUploads: true,
      done: function (e, data) {
          attachments = attachments.concat(data.result);
          refreshAttachments();
      },
      add: function(e, data) {
          var file = data.files[0];
          data.context =
              $('&lt;li&gt;',{'class':'file-upload-item'})
                  .append($('&lt;span&gt;').text(file.name))
                  .append(
                      $('&lt;div&gt;',{'class':'progressbar'})
                      .append($('&lt;div&gt;',{'class':'progress'}))
                  ).appendTo($fileUploads);
          data.submit(); // start upload immediately
      },
      progress: function(e, data) {
          var progress = data.loaded / data.total;
          data.context.find('.progress').stop().animate({'width':(progress*100)+'%'},100,'linear');
          //data.context.find('.progress').css({'width':(progress*100)+'%'});
      }
  });
  </code></pre>
  
  <p>In my <code>api/combox_upload.php</code> script I <code>echo json_encode($_FILES)</code> and half the time it comes back blank (I'm watching the XHR request responses in Chrome developer toolbar).</p>
  
  <p>Why is that? How do I fix it so it always submits the file?</p>
  
  <p>Edit: It seems to happen more frequently with larger files.</p>
  
  <p>Could it be an issue with PHP not handling multipart data correctly? I noticed the XHR request comes in immediately, as soon as the file upload begins, but PHP obviously hasn't gotten the whole file yet... so what does it do? Does it block when I try to access the <code>$_FILES</code> object or does it just give me an empty array? Do I have to something special?</p>