Partially revert "Upgrade mail gem to remove dependency on mime-types."

Still seeing heap_live_count spikes.

This reverts commit 58b8ea4f41.

Gemfile

@@ -25,6 +25,7 @@ else
 end
 
 gem 'mail'
+gem 'mime-types', require: 'mime/types/columnar'
 gem 'mini_mime'
 gem 'mini_suffix'
 

Gemfile.lock

@@ -163,13 +163,16 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     lru_redux (1.1.0)
-    mail (2.7.0)
-      mini_mime (>= 0.1.1)
+    mail (2.6.6)
+      mime-types (>= 1.16, < 4)
     memory_profiler (0.9.10)
     message_bus (2.1.2)
       rack (>= 1.1.3)
     metaclass (0.0.4)
     method_source (0.8.2)
+    mime-types (3.1)
+      mime-types-data (~> 3.2015)
+    mime-types-data (3.2016.0521)
     mini_mime (0.1.3)
     mini_portile2 (2.3.0)
     mini_racer (0.1.15)
@@ -442,6 +445,7 @@ DEPENDENCIES
   mail
   memory_profiler
   message_bus
+  mime-types
   mini_mime
   mini_racer
   mini_suffix