title: What's so special about this PNG file?
body: <p>This PNG <a href="https://dl.dropboxusercontent.com/u/130266/test.png" rel="nofollow">file</a> can not be uploaded from my app to a 3d-party server. It always reports this error:</p>
  
  <blockquote>
    <p>does multipart has image?</p>
  </blockquote>
  
  <p>I'm sure multipart encoding is correct. Tens of thousands of images are uploaded from my app without this issue. It it the first time.</p>
  
  <p>I guess there is something special about this PNG file and I proved it:</p>
  
  <ol>
  <li>Dropbox iOS app can not display the image.</li>
  <li>Tweetbot can not upload it. The error message is "media type unrecognized".</li>
  </ol>
  
  <p>So this PNG file is indeed special and quite some apps and servers don't handle it properly. But I don't know what's so special about it and hope someone who know PNG better than me can help. Thanks.</p>