add the behaviour, work on actually making it work.

lib/pleroma/uploaders/local.ex

@@ -3,7 +3,7 @@ defmodule Pleroma.Uploaders.Local do
 
   alias Pleroma.Web
 
-  def put_file(name, uuid, file, _content_type, should_dedupe) do
+  def put_file(name, uuid, tmpfile, _content_type, should_dedupe) do
     upload_folder = get_upload_path(uuid, should_dedupe)
     url_path = get_url(name, uuid, should_dedupe)
 
@@ -12,9 +12,9 @@ defmodule Pleroma.Uploaders.Local do
     result_file = Path.join(upload_folder, name)
 
     if File.exists?(result_file) do
-      File.rm!(file.path)
+      File.rm!(tmpfile)
     else
-      File.cp!(file.path, result_file)
+      File.cp!(tmpfile, result_file)
     end
 
     url_path

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