diff --git a/.gitignore b/.gitignore
index bb18dc22..8b3949f7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@
 /.elixir_ls
 erl_crash.dump
 *.ez
+.DS_Store
diff --git a/lib/bamboo/adapters/mailgun_adapter.ex b/lib/bamboo/adapters/mailgun_adapter.ex
index b346065f..263f69ac 100644
--- a/lib/bamboo/adapters/mailgun_adapter.ex
+++ b/lib/bamboo/adapters/mailgun_adapter.ex
@@ -248,11 +248,18 @@ defmodule Bamboo.MailgunAdapter do
     Map.put(body, :attachments, attachment_data)
   end
 
-  defp prepare_file(%Attachment{} = attachment) do
+  defp prepare_file(%Attachment{content_id: nil} = attachment) do
     {"", attachment.data,
      {"form-data", [{"name", ~s/"attachment"/}, {"filename", ~s/"#{attachment.filename}"/}]}, []}
   end
 
+  # At the time of writing, Mailgun did not support clients specifying content id.
+  # Instead, they set the content id to be the given filename.
+  defp prepare_file(%Attachment{} = attachment) do
+    {"", attachment.data,
+     {"form-data", [{"name", ~s/"inline"/}, {"filename", ~s/"#{attachment.filename}"/}]}, []}
+  end
+
   @mailgun_message_fields ~w(from to cc bcc subject text html template recipient-variables)a
   @internal_fields ~w(attachments)a