Kill multiple slack files from your browser console. Simply paste this code when on the "https://[space].slack.com/files/[me]" page.

slack-file-killer.js

(function(){
  function killIt(id) {
    console.log(id);

    $.post('/api/files.delete', {
      file: id,
      token: boot_data.api_token
    }, function(data) {
      console.log(data);
    });
  }

  $('#files_list >  [data-file-id]').each(function(i) {
    killIt($(this).attr('data-file-id'));
  });
})();