Add Content-ID header when needed

lib/bamboo/adapters/smtp_adapter.ex

@@ -225,7 +225,19 @@ defmodule Bamboo.SMTPAdapter do
     |> add_smtp_line(text_body)
   end
 
-  defp add_attachment_header(body, %{content_type: content_type} = attachment)
+  defp add_attachment_header(body, attachment) do
+    case attachment.content_id do
+      nil ->
+        add_common_attachment_header(body, attachment)
+
+      cid ->
+        body
+        |> add_common_attachment_header(attachment)
+        |> add_smtp_line("Content-ID: <#{cid}>")
+    end
+  end
+
+  defp add_common_attachment_header(body, %{content_type: content_type} = attachment)
        when content_type == "message/rfc822" do
     <<random::size(32)>> = :crypto.strong_rand_bytes(4)
 
@@ -235,7 +247,7 @@ defmodule Bamboo.SMTPAdapter do
     |> add_smtp_line("X-Attachment-Id: #{random}")
   end
 
-  defp add_attachment_header(body, attachment) do
+  defp add_common_attachment_header(body, attachment) do
     <<random::size(32)>> = :crypto.strong_rand_bytes(4)
 
     body

test/attachments/attachment_one.txt

@@ -0,0 +1 @@
+Attachment One.

test/attachments/attachment_two.txt

@@ -0,0 +1 @@
+Attachment Two.

test/lib/bamboo/adapters/smtp_adapter_test.exs

@@ -730,6 +730,28 @@ defmodule Bamboo.SMTPAdapterTest do
     assert_configuration(bamboo_config, gen_smtp_config)
   end
 
+  test "email have a Content-ID properly set when attaching files with content_id" do
+    bamboo_email =
+      new_email()
+      |> Bamboo.Email.put_attachment(Path.absname("test/attachments/attachment_one.txt"),
+        content_id: "12345"
+      )
+      |> Bamboo.Email.put_attachment(Path.absname("test/attachments/attachment_two.txt"),
+        content_id: "54321"
+      )
+
+    bamboo_config = configuration()
+
+    {:ok, "200 Ok 1234567890"} = SMTPAdapter.deliver(bamboo_email, bamboo_config)
+
+    assert 1 = length(FakeGenSMTP.fetch_sent_emails())
+
+    [{{_from, _to, raw_email}, _gen_smtp_config}] = FakeGenSMTP.fetch_sent_emails()
+
+    assert Regex.run(~r{Content-ID: <12345>\r\n}, raw_email, capture: :all_but_first)
+    assert Regex.run(~r{Content-ID: <54321>\r\n}, raw_email, capture: :all_but_first)
+  end
+
   test "check rfc822 encoding for subject" do
     bamboo_email =
       @email_in_utf8