Fix software design suggestions

lib/service/activity_pub/converter/picture.ex

@@ -0,0 +1,28 @@
+defmodule Mobilizon.Service.ActivityPub.Converter.Picture do
+  @moduledoc """
+  Picture converter.
+
+  This module allows to convert events from ActivityStream format to our own
+  internal one, and back.
+  """
+
+  alias Mobilizon.Media.Picture, as: PictureModel
+
+  @doc """
+  Convert a picture struct to an ActivityStream representation.
+  """
+  @spec model_to_as(PictureModel.t()) :: map
+  def model_to_as(%PictureModel{file: file}) do
+    %{
+      "type" => "Document",
+      "url" => [
+        %{
+          "type" => "Link",
+          "mediaType" => file.content_type,
+          "href" => file.url
+        }
+      ],
+      "name" => file.name
+    }
+  end
+end