bootstrap4.css
.was-validated .custom-file-input:valid ~ .custom-file-label, .custom-file-input.is-valid ~ .custom-file-label {
  border-color: #28a745;
}
bootstrap4.css
.was-validated .custom-file-input:valid ~ .valid-feedback,
.was-validated .custom-file-input:valid ~ .valid-tooltip, .custom-file-input.is-valid ~ .valid-feedback,
.custom-file-input.is-valid ~ .valid-tooltip {
  display: block;
}
bootstrap4.css
.was-validated .custom-file-input:valid:focus ~ .custom-file-label, .custom-file-input.is-valid:focus ~ .custom-file-label {
  border-color: #28a745;
  box-shadow: 0 0 0 0.2rem rgba(40, 167, 69, 0.25);
}
bootstrap4.css
.was-validated .custom-file-input:invalid ~ .custom-file-label, .custom-file-input.is-invalid ~ .custom-file-label {
  border-color: #dc3545;
}
bootstrap4.css
.was-validated .custom-file-input:invalid ~ .invalid-feedback,
.was-validated .custom-file-input:invalid ~ .invalid-tooltip, .custom-file-input.is-invalid ~ .invalid-feedback,
.custom-file-input.is-invalid ~ .invalid-tooltip {
  display: block;
}
bootstrap4.css
.was-validated .custom-file-input:invalid:focus ~ .custom-file-label, .custom-file-input.is-invalid:focus ~ .custom-file-label {
  border-color: #dc3545;
  box-shadow: 0 0 0 0.2rem rgba(220, 53, 69, 0.25);
}
bootstrap4.css
.input-group > .form-control,
.input-group > .form-control-plaintext,
.input-group > .custom-select,
.input-group > .custom-file {
  position: relative;
  -ms-flex: 1 1 auto;
  flex: 1 1 auto;
  width: 1%;
  margin-bottom: 0;
}
bootstrap4.css
.input-group > .form-control + .form-control,
.input-group > .form-control + .custom-select,
.input-group > .form-control + .custom-file,
.input-group > .form-control-plaintext + .form-control,
.input-group > .form-control-plaintext + .custom-select,
.input-group > .form-control-plaintext + .custom-file,
.input-group > .custom-select + .form-control,
.input-group > .custom-select + .custom-select,
.input-group > .custom-select + .custom-file,
.input-group > .custom-file + .form-control,
.input-group > .custom-file + .custom-select,
.input-group > .custom-file + .custom-file {
  margin-left: -1px;
}
bootstrap4.css
.input-group > .form-control:focus,
.input-group > .custom-select:focus,
.input-group > .custom-file .custom-file-input:focus ~ .custom-file-label {
  z-index: 3;
}
bootstrap4.css
.input-group > .custom-file .custom-file-input:focus {
  z-index: 4;
}
bootstrap4.css
.input-group > .custom-file {
  display: -ms-flexbox;
  display: flex;
  -ms-flex-align: center;
  align-items: center;
}
bootstrap4.css
.input-group > .custom-file:not(:last-child) .custom-file-label,
.input-group > .custom-file:not(:last-child) .custom-file-label::after {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
bootstrap4.css
.input-group > .custom-file:not(:first-child) .custom-file-label {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.custom-file {
  position: relative;
  display: inline-block;
  width: 100%;
  height: calc(1.5em + 0.75rem + 2px);
  margin-bottom: 0;
}
bootstrap4.css
.custom-file-input {
  position: relative;
  z-index: 2;
  width: 100%;
  height: calc(1.5em + 0.75rem + 2px);
  margin: 0;
  opacity: 0;
}
bootstrap4.css
.custom-file-input:focus ~ .custom-file-label {
  border-color: #80bdff;
  box-shadow: 0 0 0 0.2rem rgba(0, 123, 255, 0.25);
}
bootstrap4.css
.custom-file-input:disabled ~ .custom-file-label {
  background-color: #e9ecef;
}
bootstrap4.css
.custom-file-input:lang(en) ~ .custom-file-label::after {
  content: "Browse";
}
bootstrap4.css
.custom-file-input ~ .custom-file-label[data-browse]::after {
  content: attr(data-browse);
}
bootstrap4.css
.custom-file-label {
  position: absolute;
  top: 0;
  right: 0;
  left: 0;
  z-index: 1;
  height: calc(1.5em + 0.75rem + 2px);
  padding: 0.375rem 0.75rem;
  font-weight: 400;
  line-height: 1.5;
  color: #495057;
  background-color: #fff;
  border: 1px solid #ced4da;
  border-radius: 0.25rem;
}
bootstrap4.css
.custom-file-label::after {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  z-index: 3;
  display: block;
  height: calc(1.5em + 0.75rem);
  padding: 0.375rem 0.75rem;
  line-height: 1.5;
  color: #495057;
  content: "Browse";
  background-color: #e9ecef;
  border-left: inherit;
  border-radius: 0 0.25rem 0.25rem 0;
}
bootstrap4.css
.custom-control-label::before,
.custom-file-label,
.custom-select {
  transition: background-color 0.15s ease-in-out, border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
}
bootstrap4.css
@media (prefers-reduced-motion: reduce) {
  .custom-control-label::before,
  .custom-file-label,
  .custom-select {
    transition: none;
  }
}
■フォーム:form-group:コンポーネント:ファイル選択

<div class="container">
  <form>
    <div class="default-file mb-5">
      <input type="file" id="dafaultFile" lang="ja">
      <label for="dafaultFile">ファイル選択...</label>
    </div>
    <div class="custom-file">
      <input type="file" class="custom-file-input" id="customFile" lang="ja">
      <label class="custom-file-label" for="customFile">ファイル選択...</label>
    </div>
  </form>