uploads: factor out image dimension and filetype detection code.

* Add MediaFile abstraction. A MediaFile represents an image or video file.
* Move filetype detection and dimension parsing code from uploads to MediaFile.

app/logical/media_file/video.rb

@@ -0,0 +1,9 @@
+class MediaFile::Video < MediaFile
+  def dimensions
+    [video.width, video.height]
+  end
+
+  def video
+    @video ||= FFMPEG::Movie.new(file.path)
+  end
+end