Issue centering form inside body | HTML - html

Question

I currently have this as part of my script, the header and 2 paragraphs are being centered properly, but container and button are off to the left side. I have tried using <center> </center> but I don't know where to properly apply them for the form and button to be centered.
Screenshot of what needs to be centered:
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js"></script>
<div class="jumbotron text-center">
<h1>Header Text</h1>
<p>Paragraph</p>
<p>Paragraph 2</p>
</div>
<div class="container">
<form action="excel-script.php" method="post" enctype="multipart/form-data">
<div class="row">
<div class="col-md-4">
<div class="form-group">
<input type="file" name="excelDoc" id="excelDoc" class="form-control" />
</div>
</div>
<div class="col-md-4">
<input type="submit" name="uploadBtn" id="uploadBtn" value="Button" class="btn btn-success" />
</div>
</div>
</form>
</div>

Answer 1

Add 'text-center' class to the row and edit the 'col' classes to have 12 as a sum
<div class="container" >
<form action="excel-script.php" method="post" enctype="multipart/form-data">
<div class="row text-center">
<div class="col-md-2"></div>
<div class="col-md-4">
<div class="form-group">
<input type="file" name="excelDoc" id="excelDoc" class="form-control" />
</div>
</div>
<div class="col-md-3">
<input type="submit" name="uploadBtn" id="uploadBtn" value="Button" class="btn btn-success" />
</div>
<div class="col-md-3"></div>
</div>
</form>
</div>

Answer 2

Check out this code:
<div class="text-center">
<h1>Header Text</h1>
<p>Paragraph</p>
<p>Paragraph 2</p>
</div>
<div class="container">
<form action="excel-script.php" method="post" enctype="multipart/form-data">
<div class="row align-items-center justify-content-center">
<div class="col-12 col-md-4">
<div class="custom-file" id="customFile" lang="es">
<input type="file" class="custom-file-input">
<label class="custom-file-label" for="exampleInputFile">
Choose file...
</label>
</div>
</div>
<div class="col-12 col-md-1">
<input type="submit" name="uploadBtn" id="uploadBtn" value="Button" class="btn btn-success" />
</div>
</div>
</form>
</div>
Hope this helps.