Unable to download email attachment via Bamboo SentEmailViewerPlug

I’m sending out emails with .zip file(binary) attachments but I am unable to download the attachment.

I have the local adapter set up.

config :Test, Test.Mailer,
  adapter: Bamboo.LocalAdapter,
  open_email_in_browser_url: "http://localhost.test:4000/sent_emails"

I also have the route set up.

if Mix.env == :dev do
    forward "/sent_emails", Bamboo.SentEmailViewerPlug

Everything gets forwarded to the sent_emails route and I see the attachment to the email but when I click on the link for the attachment I get…

no function clause matching in Plug.Conn.put_resp_content_type/3

def put_resp_content_type(conn, content_type, nil) when is_binary(content_type) do
    put_resp_header(conn, "content-type", content_type)