config: allow for accepted post formats to be configured

lib/pleroma/web/common_api/common_api.ex

@@ -73,6 +73,11 @@ defmodule Pleroma.Web.CommonAPI do
   def get_visibility(_), do: "public"
 
   @instance Application.get_env(:pleroma, :instance)
+  @allowed_post_formats Keyword.get(@instance, :allowed_post_formats)
+
+  defp get_content_type(content_type) when content_type in @allowed_post_formats, do: content_type
+  defp get_content_type(_), do: "text/plain"
+
   @limit Keyword.get(@instance, :limit)
   def post(user, %{"status" => status} = data) do
     visibility = get_visibility(data)
@@ -90,7 +95,7 @@ defmodule Pleroma.Web.CommonAPI do
              mentions,
              attachments,
              tags,
-             data["content_type"] || "text/plain",
+             get_content_type(data["content_type"]),
              data["no_attachment_links"]
            ),
          context <- make_context(inReplyTo),