application controller: fix bad file extension errors.

Fix requests with bad file extensions not always returning errors correctly:

* https://danbooru.donmai.us/posts.jpg
* https://danbooru.donmai.us/posts.blah
* https://danbooru.donmai.us/posts/bad.jpg
* https://danbooru.donmai.us/posts/bad.blah

test/functional/application_controller_test.rb

@@ -0,0 +1,39 @@
+require "test_helper"
+
+class ApplicationControllerTest < ActionDispatch::IntegrationTest
+  context "The application controller" do
+    should "return 406 Not Acceptable for a bad file extension" do
+      get posts_path, params: { format: :jpg }
+      assert_response 406
+
+      get posts_path, params: { format: :blah }
+      assert_response 406
+    end
+
+    context "on a RecordNotFound error" do
+      should "return 404 Not Found even with a bad file extension" do
+        get post_path("bad.json")
+        assert_response 404
+
+        get post_path("bad.jpg")
+        assert_response 404
+
+        get post_path("bad.blah")
+        assert_response 404
+      end
+    end
+
+    context "on a PaginationError" do
+      should "return 410 Gone even with a bad file extension" do
+        get posts_path, params: { page: 999999999 }, as: :json
+        assert_response 410
+
+        get posts_path, params: { page: 999999999 }, as: :jpg
+        assert_response 410
+
+        get posts_path, params: { page: 999999999 }, as: :blah
+        assert_response 410
+      end
+    end
+  end
+end