Convert style name to symbol for the interpolator

Fixes #745

lib/paperclip/interpolations.rb

@@ -93,7 +93,7 @@ module Paperclip
     # If the style has a format defined, it will return the format instead
     # of the actual extension.
     def extension attachment, style_name
-      ((style = attachment.styles[style_name]) && style[:format]) ||
+      ((style = attachment.styles[style_name.to_sym]) && style[:format]) ||
         File.extname(attachment.original_filename).gsub(/^\.+/, "")
     end
 

test/interpolations_test.rb

@@ -46,8 +46,11 @@ class InterpolationsTest < Test::Unit::TestCase
   should "return the extension of the file as the format if defined in the style" do
     attachment = mock
     attachment.expects(:original_filename).never
-    attachment.expects(:styles).returns({:style => {:format => "png"}})
-    assert_equal "png", Paperclip::Interpolations.extension(attachment, :style)
+    attachment.expects(:styles).twice.returns({:style => {:format => "png"}})
+
+    [:style, 'style'].each do |style|
+      assert_equal "png", Paperclip::Interpolations.extension(attachment, style)
+    end
   end
 
   should "return the extension of the file based on the content type" do