uploads: factor out remaining image methods to MediaFile.

app/logical/media_file/video.rb

@@ -1,10 +1,12 @@
 class MediaFile::Video < MediaFile
-  extend Memoist
-
   def dimensions
     [video.width, video.height]
   end
 
+  def duration
+    video.duration
+  end
+
   def preview(max_width, max_height)
     preview_frame.preview(max_width, max_height)
   end
@@ -13,6 +15,10 @@ class MediaFile::Video < MediaFile
     preview_frame.crop(max_width, max_height)
   end
 
+  def has_audio?
+    video.audio_channels.present?
+  end
+
   private
 
   def video