discourse/app/assets/stylesheets/mobile/upload.scss

.upload-selector {
  input[type="text"]{
    width: 520px;
  }
  input[type="file"] {
    font-size: 1em;
    line-height: 18px;
  }
  .description {
    color: dark-light-choose($primary-medium, $secondary-medium);
  }
}

.uploaded-image-preview {
  height: 150px;
}