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.rb

@@ -0,0 +1,77 @@
+class MediaFile
+  extend Memoist
+  attr_accessor :file
+
+  # delegate all File methods to `file`.
+  delegate *(File.instance_methods - MediaFile.instance_methods), to: :file
+
+  def self.open(file)
+    file = Kernel.open(file, "r", binmode: true) unless file.respond_to?(:read)
+
+    case file_ext(file)
+    when :jpg, :gif, :png
+      MediaFile::Image.new(file)
+    when :swf
+      MediaFile::Flash.new(file)
+    when :webm, :mp4
+      MediaFile::Video.new(file)
+    when :zip
+      MediaFile::Ugoira.new(file)
+    else
+      MediaFile.new(file)
+    end
+  end
+
+  def self.file_ext(file)
+    header = file.pread(16, 0)
+
+    case header
+    when /\A\xff\xd8/n
+      :jpg
+    when /\AGIF87a/, /\AGIF89a/
+      :gif
+    when /\A\x89PNG\r\n\x1a\n/n
+      :png
+    when /\ACWS/, /\AFWS/, /\AZWS/
+      :swf
+    when /\x1a\x45\xdf\xa3/n
+      :webm
+    when /\A....ftyp(?:isom|3gp5|mp42|MSNV|avc1)/
+      :mp4
+    when /\APK\x03\x04/
+      :zip
+    else
+      :bin
+    end
+  end
+
+  def initialize(file)
+    @file = file
+  end
+
+  def dimensions
+    [0, 0]
+  end
+
+  def width
+    dimensions.first
+  end
+
+  def height
+    dimensions.second
+  end
+
+  def md5
+    Digest::MD5.file(file.path).hexdigest
+  end
+
+  def file_ext
+    MediaFile.file_ext(file)
+  end
+
+  def file_size
+    file.size
+  end
+
+  memoize :dimensions, :file_ext, :file_size, :md5
+end