The Mail Resend button is not working

This was implemented in XWIKI-11486. It's not working because it's using a 'multipart/form-data' encoding type and a POST. Thus the parameters won't be available in the request but will need to be parsed from the body. See https://stackoverflow.com/a/5227/153102 & https://balusc.omnifaces.org/2009/12/uploading-files-in-servlet-30.html