adding new generator

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

lib/thumbnail/strategies/pdf_thumbnail_generator.rb

+module Thumbnail
+  module Strategies
+    class PdfThumbnailGenerator < ::Thumbnail::GeneratableStrategy
+
+      def generate(media)
+        pdf = first_page(media.path)
+        hash = SecureRandom.hex(10)
+        output = "/tmp/#{hash}.png"
+        pdf.write(output)
+        File.open output
+      end
+
+      def can_generate?(media)
+        MimeMagic.by_magic(media).type.include? "pdf"
+      end
+
+      private
+
+      def first_page(pdf_path)
+        first_page_path = pdf_path + "[0]"
+        Magick::Image.read(first_page_path).first
+      end
+
+    end
+  end
+end
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