diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index e0ac6c24b1c762627982ee84fd534e67a56589fb..0ca3d2eb015b3a2fac6591fa4a63fde5b77cb07e 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Add extension synonyms `yml` and `yaml` for MIME type `application/x-yaml`.
+
+   *bogdanvlviv*
+
 *   Adds support for including ActionController::Cookies in API controllers.
     Previously, including the module would raise when trying to define
     a `cookies` helper method. Skip calling #helper_method if it is not
diff --git a/actionpack/lib/action_dispatch/http/mime_types.rb b/actionpack/lib/action_dispatch/http/mime_types.rb
index 66cea88256ec9f4e451013b11f1fbf7deafc67fb..8b04174f1feb6230e10bc6b1eb23a6ae24c7a8ba 100644
--- a/actionpack/lib/action_dispatch/http/mime_types.rb
+++ b/actionpack/lib/action_dispatch/http/mime_types.rb
@@ -21,7 +21,7 @@
 Mime::Type.register "application/xml", :xml, %w( text/xml application/x-xml )
 Mime::Type.register "application/rss+xml", :rss
 Mime::Type.register "application/atom+xml", :atom
-Mime::Type.register "application/x-yaml", :yaml, %w( text/yaml )
+Mime::Type.register "application/x-yaml", :yaml, %w( text/yaml ), %w(yml yaml)
 
 Mime::Type.register "multipart/form-data", :multipart_form
 Mime::Type.register "application/x-www-form-urlencoded", :url_encoded_form