title: File upload field is reset when submit form
body: <p>I am using ExtJS version 4.1.1.</p>
  
  <p>I try to create upload file form using filefield as follow:</p>
  
  <pre><code>{
      xtype: 'filefield',
      itemId : 'my-file',
      name: 'my file',
      emptyText : 'No file chosen',
      fieldLabel: 'Upload File',
      submitValue: true,
      allowBlank : false,
      buttonText: 'Browse',
      listeners: {
          change: function(fld, value) {
              var newValue = value.replace(/C:\\fakepath\\/g, '');
              fld.setRawValue(newValue);
          }
      }
  }
  </code></pre>
  
  <p>When the form is submitted, the <code>filefield</code> is reset.</p>
  
  <p>As I see in
  <a href="http://www.sencha.com/forum/showthread.php?135109-File-upload-field-is-empty-by-re-submitting-of-the-form" rel="nofollow">http://www.sencha.com/forum/showthread.php?135109-File-upload-field-is-empty-by-re-submitting-of-the-form</a></p>
  
  <p>I try to override the <code>filefield</code> to :</p>
  
  <pre><code>Ext.override(Ext.form.field.File, {
      extractFileInput: function() {
          var me = this,
              fileInput = me.fileInputEl.dom,
              clone = fileInput.cloneNode(true);
  
          fileInput.parentNode.replaceChild(clone, fileInput);
          me.fileInputEl = Ext.get(clone);
  
          me.fileInputEl.on({
              scope: me,
              change: me.onFileChange
          });
  
          return fileInput;
  }
  </code></pre>
  
  <p>It look OK when I submit the form.</p>
  
  <p>The value that I see in the textfield is not reset to empty.</p>
  
  <p>However, when I submit the form again without re-choose file, the data that be sent to the server is null.</p>
  
  <p>The data that be sent to the server should be retained.</p>
  
  <p><strong>Additional Info:</strong></p>
  
  <p>This problem occur when I use Chrome and IE, It seem work fine on Firefox.</p>
  
  <p>Is it related with <code>C:\\fakepath</code> that I see on textfield when choose file?</p>
  
  <p>How can I fix this problem?</p>