title: "Open in" feature in UIActivityViewController
body: <p>I need your help with <code>UIActivityViewController</code>.
  I need to present activity controller for the file where, except for default actions, the user will be able to open the file in other apps.
  I know how to do this with <code>UIDocumentController</code>, which shows popover with all apps able to open the file.
  But how can I show all these apps in <code>UIActivityViewController</code>?</p>
  
  <p>P.S. This behavior can be seen in Apple Mail app, for example.</p>