Add mime types gem for AWS

Also sorted the Gemfile.

Gemfile

@@ -8,15 +8,26 @@ gem 'rails', '~> 5.1'
 # Bundle edge Rails instead:
 # gem 'rails', :git => 'git://github.com/rails/rails.git'
 
-gem 'sass-rails'
+gem 'administrate'
-gem 'coffee-rails'
+gem 'administrate-field-image'
-gem 'uglifier'
-
-gem 'bourbon'
-
-gem 'jquery-rails'
-
 gem 'aws-sdk'
+gem 'bourbon'
+gem 'coffee-rails'
+gem 'dragonfly'
+gem 'dragonfly-s3_data_store'
+gem 'jquery-rails'
+gem 'kaminari'
+gem 'kramdown'
+gem 'mime-types'
+gem 'nokogiri'
+gem 'oath'
+gem 'oath-generators'
+gem 'pg'
+gem 'sass-rails'
+gem 'simple_form'
+gem 'slugtastic'
+gem 'uglifier'
+gem 'unf'
 
 # Bundle gems for the local environment. Make sure to
 # put test-only gems in this group so their generators
@@ -50,17 +61,3 @@ group :production do
   gem 'puma'
   gem 'rails_12factor'
 end
-
-gem 'administrate'
-gem 'administrate-field-image'
-gem 'dragonfly'
-gem 'dragonfly-s3_data_store'
-gem 'kaminari'
-gem 'oath'
-gem 'oath-generators'
-gem 'nokogiri'
-gem 'pg'
-gem 'kramdown'
-gem 'simple_form'
-gem 'slugtastic'
-gem 'unf'

Gemfile.lock

@@ -175,6 +175,9 @@ GEM
     mail (2.7.0)
       mini_mime (>= 0.1.1)
     method_source (0.9.0)
+    mime-types (3.1)
+      mime-types-data (~> 3.2015)
+    mime-types-data (3.2016.0521)
     mini_mime (1.0.0)
     mini_portile2 (2.3.0)
     minitest (5.11.3)
@@ -349,6 +352,7 @@ DEPENDENCIES
   kaminari
   kramdown
   letter_opener
+  mime-types
   nokogiri
   oath
   oath-generators