diff --git a/Gemfile b/Gemfile
index d9ec1e8..ccfc696 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,6 +5,7 @@ gem 'therubyracer', '~> 0.12.0'
 gem 'less',         '~>  2.6.0'
 gem 'html-proofer', '~>  2.3.0'
 gem 'colorize',     '~>  0.7.0'
+gem 'mime-types' # Required for middleman-s3_sync
 
 gem 'middleman-s3_sync',    '~> 3.0.0'
 gem 'middleman',            '~> 3.2.0'