Add stubs for audio preview to attachment approval dialog.

// FREEBIE

Signal/src/ViewControllers/AttachmentApprovalViewController.swift

@@ -69,11 +69,18 @@ class AttachmentApprovalViewController: UIViewController {
             createImagePreview(attachmentPreviewView:attachmentPreviewView)
         } else if attachment.isVideo {
             createVideoPreview(attachmentPreviewView:attachmentPreviewView)
+        } else if attachment.isAudio {
+            createAudioPreview(attachmentPreviewView:attachmentPreviewView)
         } else {
             createGenericPreview(attachmentPreviewView:attachmentPreviewView)
         }
     }
 
+    private func createAudioPreview(attachmentPreviewView: UIView) {
+        // TODO: Add audio player.
+        createGenericPreview(attachmentPreviewView:attachmentPreviewView)
+    }
+
     private func createAnimatedPreview(attachmentPreviewView: UIView) {
         // Use Flipboard FLAnimatedImage library to display gifs
         guard let animatedImage = FLAnimatedImage(gifData:attachment.data) else {