Change module_invoke_all to drupal_alter for filemode_signature

I'm trying to add test to my own module that depend on the RESTClient module with file mode to have static data. Since all of my request contain the current timestamp in the header, the file name is always unique. Changing the call to module_invoke_all with a call to drupal_alter would let me implement the alter hook to remove the header from the options to generate a file name that does not change for every request.