title: Conditional cast from PFFile toPFFile always succeed
body: <p>I get a warming of "Conditional cast from PFFile toPFFile always succeed" which highlighted <code>if let tempProductPicture = self.askProductImageArray[0] as? PFFile</code>. What is the best way to solve it? Thanks</p>
  
  <pre><code>var askProductImageArray = [PFFile]()
  override func viewDidLoad() {
      let query = PFQuery(className: "products")
      query.whereKey("title", equalTo: labelTitleText)
      query.findObjectsInBackgroundWithBlock { (objects: [PFObject]?, error:NSError?) -&gt; Void in
              if error != nil {
              }
              for object in objects! {
                  self.askProductImageArray.append(object.objectForKey("detailsImage") as! PFFile)
                  self.askTable.reloadData()
              }
              if let tempProductPicture = self.askProductImageArray[0] as? PFFile {
                  tempProductPicture.getDataInBackgroundWithBlock { data, error in
                      if data == nil {
  
                      } else if error != nil {
                      } else {
                          self.productPicture.image = UIImage(data: data!)
                      }
                  }
              }
  }
  </code></pre>