#file-receiver {
  display: block;
  margin: 1em auto;
  background: #eee;
  width: 300px;
  height: 300px;
  background-size: cover;
  background-repeat: no-repeat;
  background-position: center;
}
#file-receiver input#file {
  width: 0px;
  height: 0px;
  visibility: hidden;
}
#file-receiver.walking-ants {
  outline: 1px dashed black;
}