Add changelog for custom content type

actionmailer/CHANGELOG.md

@@ -3,4 +3,16 @@
 
     *Jeremy Daer*
 
+*   Mime type: allow to custom content type when setting body in headers
+    and attachments.
+
+    Example:
+
+        def test_emails
+          attachments["invoice.pdf"] = "This is test File content"
+          mail(body: "Hello there", content_type: "text/html")
+        end
+
+    *Minh Quy*
+
 Please check [5-0-stable](https://github.com/rails/rails/blob/5-0-stable/actionmailer/CHANGELOG.md) for previous changes.

guides/source/5_0_release_notes.md

@@ -533,6 +533,10 @@ Please refer to the [Changelog][action-mailer] for detailed changes.
     whether your templates should perform caching or not.
     ([Pull Request](https://github.com/rails/rails/pull/22825))
 
+*   Added support for custom `content_type` when setting body in
+    headers and attachments.
+    ([Pull Request](https://github.com/rails/rails/pull/27227))
+
 
 Active Record
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