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

.upload-selector {
  input[type="text"]{
    width: 520px;
  }
  input[type="file"] {
    font-size: 14px;
    line-height: 18px;
  }
  .description {
    color: lighten($primary_text_color, 35%);
  }
}