title: iconForFile] returns image with 0 representations
body: <p>My Cocoa application displays file icons for all files in a given folder. I'm using this code to get the icon images:</p>
  
  <pre><code>NSImage *icon = [[NSWorkspace sharedWorkspace] iconForFile:thePath];
  </code></pre>
  
  <p>Once in a while, the returned icon has no (0) image representations, but a size set to 32x32 (default size as the documentation says), other times the correct icon (as shown in Finder) is returned.</p>
  
  <p>I used to check for <code>nil</code>, but my app would crash later until I discovered that the image was "empty', so to speak. </p>
  
  <p>When it happens, it is always the same file: a Safari <code>.webarchive</code> (the folder has two webarchives, the icon shows for the first one...). </p>
  
  <p>Is there something special about the code above? This is running in a background thread. Could this be the problem? I can't imagine why it should be a problem, as no UI elements are updated from that thread.</p>
  
  <p>Thanks! Mark.</p>