diff --git a/middleman-core/lib/middleman-core/core_extensions/request.rb b/middleman-core/lib/middleman-core/core_extensions/request.rb
index aabb708e..612001e1 100644
--- a/middleman-core/lib/middleman-core/core_extensions/request.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/request.rb
@@ -21,6 +21,9 @@ module Middleman
           ::Rack::Mime::MIME_TYPES['.html'] = 'text/html; charset=utf-8'
           ::Rack::Mime::MIME_TYPES['.htm'] = 'text/html; charset=utf-8'
 
+          # Sourcemap format
+          ::Rack::Mime::MIME_TYPES['.map'] = 'application/json; charset=utf-8'
+
           app.extend ClassMethods
           app.extend ServerMethods