Added a feature to evaluate lambdas for fog_file properties

lib/paperclip/storage/fog.rb

@@ -66,7 +66,16 @@ module Paperclip
       end
 
       def fog_file
-        @fog_file ||= @options[:fog_file] || {}
+        @fog_file ||= begin
+          value = @options[:fog_file]
+          if !value
+            {}
+          elsif value.respond_to?(:call)
+            value.call(self)
+          else
+            value
+          end
+        end
       end
 
       def fog_public(style = default_style)

test/storage/fog_test.rb

@@ -90,6 +90,27 @@ class FogTest < Test::Unit::TestCase
       end
     end
 
+    context "with file params provided as lambda" do
+      setup do
+        fog_file = lambda{ |a| { :custom_header => a.instance.custom_method }}
+        klass = rebuild_model :storage => :fog,
+                              :fog_file => fog_file
+
+        klass.class_eval do
+          def custom_method
+            'foobar'
+          end
+        end
+
+
+        @dummy = Dummy.new
+      end
+
+      should "be able to evaluate correct values for file headers" do
+        assert_equal @dummy.avatar.send(:fog_file), { :custom_header => 'foobar' }
+      end
+    end
+
     setup do
       @fog_directory = 'papercliptests'