calculate fingerprints efficiently on large files using Digest::MD5.file()

The tests would broken for StringIO, because it doesn't have a real "path". I've modify the patch to make the test stop complaining [@sikachu]

This closes #470

lib/paperclip/attachment.rb

@@ -225,10 +225,14 @@ module Paperclip
     end
     end
 
 
     def generate_fingerprint(source)
     def generate_fingerprint(source)
+      if source.respond_to?(:path) && source.path && !source.path.blank?
+        Digest::MD5.file(source.path).to_s
+      else
         data = source.read
         data = source.read
         source.rewind if source.respond_to?(:rewind)
         source.rewind if source.respond_to?(:rewind)
         Digest::MD5.hexdigest(data)
         Digest::MD5.hexdigest(data)
       end
       end
+    end
 
 
     # Paths and URLs can have a number of variables interpolated into them
     # Paths and URLs can have a number of variables interpolated into them
     # to vary the storage location based on name, id, style, class, etc.
     # to vary the storage location based on name, id, style, class, etc.