In Bootstrap 4, the initial placeholder value is set in the custom-file-control with a pseudo CSS
Example
<div class="container">
<div class="input-group mb-3">
<div class="input-group-prepend">
<span class="input-group-text">Upload</span>
</div>
<div class="custom-file">
<input type="file" class="custom-file-input" id="inputGroupFile01">
<label class="custom-file-label" for="inputGroupFile01">Choose file</label>
</div>
</div>
<div class="input-group mb-3">
<div class="custom-file">
<input type="file" class="custom-file-input" id="inputGroupFile02">
<label class="custom-file-label" for="inputGroupFile02">Choose file</label>
</div>
<div class="input-group-append">
<span class="input-group-text" id="">Upload</span>
</div>
</div>
<div class="input-group mb-3">
<div class="input-group-prepend">
<button class="btn btn-outline-secondary" type="button">Button</button>
</div>
<div class="custom-file">
<input type="file" class="custom-file-input" id="inputGroupFile03">
<label class="custom-file-label" for="inputGroupFile03">Choose file</label>
</div>
</div>
<div class="input-group">
<div class="custom-file">
<input type="file" class="custom-file-input" id="inputGroupFile04">
<label class="custom-file-label" for="inputGroupFile04">Choose file</label>
</div>
<div class="input-group-append">
<button class="btn btn-outline-secondary" type="button">Button</button>
</div>
</div>
</div>