adding image thumbnail generator

Signed-off-by: man13 <man13@inf.ufpr.br>

lib/thumbnail/strategies/image_thumbnail_generator.rb

+module Thumbnail
+  module Strategies
+    class ImageThumbnailGenerator < ::Thumbnail::GeneratableStrategy
+
+      def generate(media)
+        media
+      end
+
+      def can_generate?(media)
+        MimeMagic.by_magic(media).type =~ /image/
+      end
+
+    end
+  end
+end
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