* `{: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.
* `{: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.