สอบถามเรื่องอัพโหลดไฟล์ อยากให้ต้องมีการเลือกไฟล์อย่างน้อย 1 input ถึงจะให้ submit ปุ่มได้

if( $('input[type="file"]).length<1){
   alert('Please select a file');
}

<form id="frm_upload" name="frm_upload" method="post" action="" enctype="multipart/form-data">

<?php for($i=0;$i<3;$i++){ ?>
	<input type="file" name="file_up[]" id="file_up">
	<br>
<? } ?>

<br>
<button type="submit" id="upload" name="upload" >upload</button>
</form>

<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script type="text/javascript">
	$(document).ready(function () {
        $("#frm_upload").on("submit", function (event) {
		event.preventDefault(); 
        var formData = new FormData($(this)[0]);
		var forms=document.frm_upload;
		var file_up = $('#file_up').val();

		//alert(file_up);
		if( $('input[type="file"]').length<1){
			alert('Please select a file');
		}else{
			alert('OK');
		}
		
		});
	});
</script>

<form id="frm_upload" name="frm_upload" method="post" action="" enctype="multipart/form-data">

<?php for($i=0;$i<3;$i++){ ?>
	<input type="file" name="file_up[]" id="file_up">
	<br>
<? } ?>

<br>
<button type="submit" id="upload" name="upload" >upload</button>
</form>

<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script type="text/javascript">
	$(document).ready(function () {
        $("#frm_upload").on("submit", function (event) {
		event.preventDefault(); 
        var formData = new FormData($(this)[0]);
		var forms=document.frm_upload;
		var file_up = $('#file_up').val();

		//alert(file_up);
	//	if( $('input[type="file"]').length<1){
		if($('input[type="file"]')[0].length<1){
			alert('Please select a file');
		}else{
			alert('OK');
		}
		
		});
	});
</script>

<form id="frm_upload" name="frm_upload" method="post" action="" enctype="multipart/form-data">

<?php for ($i=0;$i<3;$i++) { ?>
	<input type="file" name="file_up[]" id="file_up<?php echo $i; ?>">
	<br>
<?php } ?>

<br>
<button type="submit" id="upload" name="upload" >upload</button>
</form>

<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script type="text/javascript">
	$(document).ready(function () {
        $("#frm_upload").on("submit", function (event) {
            event.preventDefault(); 
            let inputFile = $('input[type="file"]');
            let selectedItems = 0;
            if (typeof(inputFile) === 'object' && inputFile.length >= 1) {
                $.each(inputFile, function(index, item) {
                    if (typeof(item.value) === 'string' && item.value !== '') {
                        selectedItems++;
                    }
                });
            }

            if (selectedItems > 0) {
                alert('OK');
            } else {
                alert('Please select a file.');
            }
		});
	});
</script>

function chk_file(){
  var i=0; ob = $('input[type="file"]');
  for( i; i<ob.length; i++){
     if( $(ob[i]).val().length<1) return false;
  }
  return true;
}

// in method event button submit
if(chk_file()){
  // do submit
}else{
  alert('Please select file in empty file upload');
}