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/flash.rb

@@ -0,0 +1,6 @@
+class MediaFile::Flash < MediaFile::Image
+  def dimensions
+    image_size = ImageSpec.new(file.path)
+    [image_size.width, image_size.height]
+  end
+end