dist/flow.js
function (event, callback) {
      event = event.toLowerCase();
      if (!this.events.hasOwnProperty(event)) {
        this.events[event] = [];
      }
      this.events[event].push(callback);
    }
function (event, callback) {
      event = event.toLowerCase();
      if (!this.events.hasOwnProperty(event)) {
        this.events[event] = [];
      }
      this.events[event].push(callback);
    }
Set a callback for an event, possible events:
fileSuccess(file), fileProgress(file), fileAdded(file, event),
fileRemoved(file), fileRetry(file), fileError(file, message),
complete(), progress(), error(message, file), pause()
@function
@param {string} event
@param {Function} callback