diff --git a/Gemfile b/Gemfile
index aecd82702d59f896063b959a6882df48d169bdb0..6ee884a17a4b3250938cd23290087db4e0351d02 100644
--- a/Gemfile
+++ b/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'
diff --git a/Gemfile.lock b/Gemfile.lock
index dac6169d5745d4b68aa8ec748dc3fc062cc25b75..f0156529ca58b66b47c1baf7230d5e4976aa3c27 100644
--- a/Gemfile.lock
+++ b/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)
diff --git a/app/models/media_attachment.rb b/app/models/media_attachment.rb
index 340109ab607c00eef94018cfd7cdb76cd8da1b20..1e8c6d00ae31445060f7a065d2b1fd050c735f0d 100644
--- a/app/models/media_attachment.rb
+++ b/app/models/media_attachment.rb
@@ -18,6 +18,8 @@
 #  file_meta         :json
 #
 
+require 'mime/types'
+
 class MediaAttachment < ApplicationRecord
   self.inheritance_column = nil