title: Add attachment image to attributed string in iOS 6
body: <p>I have spent 3 hours on this but just can't solve it, anybody knows why? please help me!</p>
  
  <p>The code below added an image as an attachment to an attributed string, </p>
  
  <pre><code>UIImage *img = [UIImage imageNamed:imgName];
  NSTextAttachment *textAttachment = [[NSTextAttachment alloc] init];
  textAttachment.image = img;
  NSAttributedString *attrStringWithImage = [NSAttributedString attributedStringWithAttachment:textAttachment];
  NSMutableAttributedString *attrStr = [[NSMutableAttributedString alloc] init];
  [attrStr appendAttributedString:attrStringWithImage];
  </code></pre>
  
  <p>Then I assigned attrStr to a UITextView's attributedText, this works great in iOS 7 but in iOS 6 the image is not displayed, I heard attributedText is supported for UITextView since iOS 6, is there anything different should I do for iOS 6?</p>
  
  <p>PS: in iOS 6, if I assign attrStr to UILabel's attributedText, it is displayed, what's the special for UITextView?</p>