post replacements: allow replacing with an uploaded file.

test/unit/post_replacement_test.rb

@@ -191,6 +191,18 @@ class PostReplacementTest < ActiveSupport::TestCase
       end
     end
 
+    context "a post with an uploaded file" do
+      should "work" do
+        Tempfile.open do |file|
+          file.write(File.read("#{Rails.root}/test/files/test.png"))
+          file.seek(0)
+
+          @post.replace!(replacement_file: file, replacement_url: "")
+          assert_equal(@post.md5, Digest::MD5.file(file).hexdigest)
+        end
+      end
+    end
+
     context "a post when given a final_source" do
       should "change the source to the final_source" do
         replacement_url = "http://data.tumblr.com/afed9f5b3c33c39dc8c967e262955de2/tumblr_orwwptNBCE1wsfqepo1_raw.png"