Explicitly require MIME::Types (#4083)

Gemfile

@@ -35,6 +35,7 @@ gem 'http_accept_language', '~> 2.1'
 gem 'httplog', '~> 0.99'
 gem 'kaminari', '~> 1.0'
 gem 'link_header', '~> 0.0'
+gem 'mime-types', '~> 3.1'
 gem 'nokogiri', '~> 1.7'
 gem 'oj', '~> 3.0'
 gem 'ostatus2', '~> 2.0'

Gemfile.lock

@@ -516,6 +516,7 @@ DEPENDENCIES
   link_header (~> 0.0)
   lograge (~> 0.5)
   microformats2 (~> 3.0)
+  mime-types (~> 3.1)
   nokogiri (~> 1.7)
   oj (~> 3.0)
   ostatus2 (~> 2.0)

app/models/media_attachment.rb

@@ -18,6 +18,8 @@
 #  file_meta         :json
 #
 
+require 'mime/types'
+
 class MediaAttachment < ApplicationRecord
   self.inheritance_column = nil