usage.js

import wait from './wait';
  //...

  save(formData) {
     //...

        upload(formData)
          .then(wait(1500)) // DEV ONLY: wait for 1.5s 
          .then(x => {
            this.uploadedFiles = [].concat(x);
            this.currentStatus = STATUS_SUCCESS;
          })
         //...
      },

wait.js

export default function wait(ms) {
    return (x) => {
        return new Promise(resolve => setTimeout(() => resolve(x), ms));
    };
}