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Bootstrap class: .custom-file

<div class="custom-file">
  <input type="file" class="custom-file-input" id="customFile">
  <label class="custom-file-label" for="customFile">Choose file</label>
</div>

Preview

Sass source

/* _custom-forms.scss:240 */
.custom-file {
  position: relative;
  display: inline-block;
  width: 100%;
  height: $custom-file-height;
  margin-bottom: 0;
}

Uses variables

/* _custom-forms.scss:240 */
$custom-file-height