There is a bug in https://github.com/moodle/moodle/blob/master/files/externallib.php

The correct data type for 'filename' is PARAM_TEXT not PARAM_FILE, 'filename' is used for the name of files and also directories

This bug causes the WebService server to raise an exception (invalid_response_exception)

This is an example of data returned by the webservice:

    array (

      'contextid' => '68',

      'component' => NULL,

      'filearea' => NULL,

      'itemid' => NULL,

      'filepath' => NULL,

      'filename' => 'An MP3 file (File)',

    ),

As you can see the filename has some specials chars like ( or )