* `{:error, String.t}` error information if the file failed to be saved to the backend.
* `:wait_callback` will wait for an http post request at `/api/pleroma/upload_callback/:upload_path` and call the uploader's `http_callback/3` method.
-
"""
@type file_spec :: {:file | :url, String.t()}
@callback put_file(Pleroma.Upload.t()) ::