wagtail/client/scss/components/forms/_file.scss

.file_field {
  .input {
    label {
      float: none;
      display: inline;
      padding: 0;
    }

    input[type='checkbox'] {
      margin-top: 5px;
    }

    a {
      &:after {
        content: ' ';
        display: block;
      }
    }
  }
}