const MULTIMEDIA_IMAGE_TYPE_ID = 12

const multimedias = [
  { mediumSizeUrl: 'dog.jpg', typeId: 2 },
  { mediumSizeUrl: 'panda.jpg', typeId: 12 },
  { mediumSizeUrl: 'koala.jpg', typeId: 12 },
  { mediumSizeUrl: 'trex.jpg', typeId: 5 },
  { mediumSizeUrl: 'ratilla.jpg', typeId: 12 }
];

const isUsable = media => media.typeId === MULTIMEDIA_IMAGE_TYPE_ID && media.mediumSizeUrl;
const images = multimedias.reduce((accumulator, media) => {
  if (isUsable(media)) accumulator.push(media.mediumSizeUrl)
  return accumulator;
}, [])

console.log(images) // ["panda.jpg", "koala.jpg", "ratilla.jpg"]