title: Which file types can UIDocumentInteractionController preview?
body: <p>I'm putting a UIDocumentInteractionController into my app, which can download arbitrary file types from a server.  </p>
  
  <p>It's there for two purposes:</p>
  
  <ol>
  <li>To allow the user to 'Open In' the file in another app that may be able to view the file.</li>
  <li>To preview files that the built-in UIDocumentInteractionController is able to preview.</li>
  </ol>
  
  <p>What I'd like to so is to only attempt to display the preview for file types that the system is able to preview.  For example, if it's a zip file, the system can't preview that so I want to go straight to the 'Open In'.</p>
  
  <p>I can't find anywhere a list of the types that are supported for preview.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>self.docController = [UIDocumentInteractionController interactionControllerWithURL:[NSURL fileURLWithPath:path]];
  [self.docController setDelegate:self];
  
  // I'd like to skip this line if the system can't preview the file.     
  
  BOOL isOpen = [self.docController presentPreviewAnimated:YES];
  if (!isOpen) {
      BOOL canOpen = [self.docController presentOpenInMenuFromRect:CGRectMake(300, 300, 100, 100)
                                                            inView:self.view
                                                          animated:NO];
      if (!canOpen) {
            // tell user to install an app that's able to open this file.
            return;
       }
  }
  </code></pre>