<li id="field_3_9" class="gfield gfield_upload w-100 field_sublabel_below field_description_below gfield_visibility_visible">
  <label class="gfield_label" for="input_3_9">upload file</label>
  <div class="ginput_container ginput_container_fileupload">
    <input type="hidden" name="MAX_FILE_SIZE" value="52428800">
    <input name="input_9" id="input_3_9" type="file" class="medium" aria-describedby="validation_message_3_9 live_validation_message_3_9 extensions_message_3_9" onchange="javascript:gformValidateFileSize( this, 52428800 );" tabindex="55">
    <span id="extensions_message_3_9" class="screen-reader-text"></span>
    <div class="validation_message" id="live_validation_message_3_9">pdf test.pdf</div>
  </div>
</li>

var inputLabel = document.querySelector('.gfield_label');
var inputWrapper = document.querySelector('.gfield_upload');
var input =  document.querySelector("input[type='file']");
var inputSuccess = document.querySelector('.validation_message');

input.addEventListener('change', () => {
  const name = input.value.split(/\|//).pop();
  const truncated = name.length > 20
    ? name.substr(name.length - 20)
    : name;

    inputSuccess.innerHTML = truncated;
});