add_file_modal.php

<div class="modal" id="addFileModal" tabindex="-1">
  <div class="modal-dialog modal-sm">
    <div class="modal-content bg-dark">
      <div class="modal-header">
        <h5 class="modal-title"><i class="fa fa-fw fa-cloud-upload-alt"></i> Upload File</h5>
        <button type="button" class="close text-white" data-dismiss="modal">
          <span>&times;</span>
        </button>
      </div>
      <form action="post.php" method="post" enctype="multipart/form-data" autocomplete="off">
        <input type="hidden" name="client_id" value="<?php echo $client_id; ?>">
        <div class="modal-body bg-white">    
          
          <div class="form-group">
            <input type="text" class="form-control" name="new_name" placeholder="New File Name (Optional)">
          </div>

          <div class="form-group">
            <input type="file" class="form-control-file" name="file">
          </div>
          
        </div>
        <div class="modal-footer bg-white">
          <button type="submit" name="add_file" class="btn btn-primary">Upload</button>
        </div>
      </form>
    </div>
  </div>
</div>