UIImageView+UIImage_Extensions.swift

func associatedObject<T: AnyObject>(
    base: AnyObject,
    key: UnsafePointer<UInt8>,
    initialiser: () -> T)
    -> T {
        if let associated = objc_getAssociatedObject(base, key)
            as? T { return associated }
        let associated = initialiser()
        objc_setAssociatedObject(base, key, associated,
            .OBJC_ASSOCIATION_RETAIN)
        return associated
}

func associateObject<T: AnyObject>(
    base: AnyObject,
    key: UnsafePointer<UInt8>,
    value: T) {
        objc_setAssociatedObject(base, key, value,
            .OBJC_ASSOCIATION_RETAIN)
}


private var fileKey: UInt8 = 0

extension UIImageView {
    
    var file:PFFile  {
        
        get {
            return associatedObject(self, key: &fileKey) { self.file }
        }
        set {
            associateObject(self, key: &fileKey, value: newValue)
            self.loadImage()
        }
    }
    
    private func loadImage()
    {
        var aiView:UIActivityIndicatorView?
        
        for view in self.subviews
        {
            if let aiv = view as? UIActivityIndicatorView
            {
                aiView = aiv
            }
        }
        aiView?.startAnimating()
        file.getDataInBackgroundWithBlock { (data:NSData?, error:NSError?) -> Void in
            if let imageDatas = data
            {
                let imageFromDatas = UIImage(data: imageDatas)
                dispatch_async(dispatch_get_main_queue()) { () -> Void in
                    aiView?.stopAnimating()
                    self.image = imageFromDatas
                }
            }
            else if let err = error
            {
                print("[ERROR] loading image in UIImageView with error: " + err.localizedDescription)
            }
        }
    }
}

extension UIImage {
    var highestQualityJPEGNSData:NSData { return UIImageJPEGRepresentation(self, 1.0)! }
    var highQualityJPEGNSData:NSData    { return UIImageJPEGRepresentation(self, 0.75)!}
    var mediumQualityJPEGNSData:NSData  { return UIImageJPEGRepresentation(self, 0.5)! }
    var lowQualityJPEGNSData:NSData     { return UIImageJPEGRepresentation(self, 0.25)!}
    var lowestQualityJPEGNSData:NSData  { return UIImageJPEGRepresentation(self, 0.0)! }
}

Also add stored property to swift extension