posts: fix exception in random:1 filetype:png.

app/models/post.rb

@@ -966,11 +966,11 @@ class Post < ApplicationRecord
       end
 
       def random_up(key)
-        where("md5 < ?", key).reorder(md5: :desc).first
+        where("posts.md5 < ?", key).reorder(md5: :desc).first
       end
 
       def random_down(key)
-        where("md5 >= ?", key).reorder(md5: :asc).first
+        where("posts.md5 >= ?", key).reorder(md5: :asc).first
       end
 
       def sample(query, sample_size)

test/unit/post_query_builder_test.rb

@@ -1270,11 +1270,12 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
     end
 
     should "return posts for the random:<N> metatag" do
-      post = create(:post)
+      post = create(:post, media_asset: build(:media_asset, file_ext: "png"))
 
       assert_tag_match([], "random:0")
       assert_tag_match([post], "random:1")
       assert_tag_match([post], "random:1000")
+      assert_tag_match([post], "random:1 filetype:png")
     end
 
     should "return posts ordered by a particular attribute" do