add the behaviour, work on actually making it work.

lib/pleroma/uploaders/uploader.ex

@@ -0,0 +1,26 @@
+defmodule Pleroma.Uploaders.Uploader do
+  @moduledoc """
+  Defines the contract to put an uploaded file to any backend.
+  """
+
+  @doc """
+  Put a file to the backend.
+
+  Returns a `String.t` containing the path of the uploaded file.
+  """
+  @callback put_file(
+              name :: String.t(),
+              uuid :: String.t(),
+              file :: File.t(),
+              content_type :: String.t(),
+              should_dedupe :: Boolean.t()
+            ) :: String.t()
+
+  @callback put_file(
+              name :: String.t(),
+              uuid :: String.t(),
+              image_data :: String.t(),
+              content_type :: String.t(),
+              should_dedupe :: String.t()
+            ) :: String.t()
+end