Merge branch 'swoosh-mailgun' into 'develop'

Fix Swoosh Mailgun support

See merge request pleroma/pleroma!4217

mix.exs

@@ -202,6 +202,7 @@ defmodule Pleroma.Mixfile do
       {:bandit, "~> 1.5.2"},
       {:websock_adapter, "~> 0.5.6"},
       {:oban_live_dashboard, "~> 0.1.1"},
+      {:multipart, "~> 0.4.0", optional: true},
 
       ## dev & test
       {:phoenix_live_reload, "~> 1.3.3", only: :dev},