Upgrade gitlab-grit gem to 2.8.1, loosening the mime-types gem's version dependency.

Resolves #15101.

Gemfile.lock

@@ -346,10 +346,10 @@ GEM
       flowdock (~> 0.7)
       gitlab-grit (>= 2.4.1)
       multi_json
-    gitlab-grit (2.7.3)
+    gitlab-grit (2.8.1)
       charlock_holmes (~> 0.6)
       diff-lcs (~> 1.1)
-      mime-types (~> 1.15)
+      mime-types (>= 1.16, < 3)
       posix-spawn (~> 0.3)
     gitlab_emoji (0.3.1)
       gemojione (~> 2.2, >= 2.2.1)
@@ -465,7 +465,7 @@ GEM
       mime-types (>= 1.16, < 4)
     mail_room (0.6.1)
     method_source (0.8.2)
-    mime-types (1.25.1)
+    mime-types (2.99.1)
     mimemagic (0.3.0)
     mini_portile2 (2.0.0)
     minitest (5.7.0)